Is an isolating transformer a surge protective component (SPC)?

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Is an isolating transformer a surge protective component (SPC)?

Is an isolating transformer a surge protective component (SPC)?
Is an isolating transformer a surge protective component (SPC)? Part 1
There’s been debate in the US about this so I thought it worthwhile to examine the component and question.
Definitions are:
isolating transformer
transformer with protective separation between the input and output windings
Published in:    IEC 60065, ed. 7.0 (2001-12)
safety isolating transformer
transformer, the input winding of which is electrically separated from the output winding by an insulation at least equivalent to double insulation or reinforced insulation, that is intended to supply an appliance or circuit at safety extra-low voltage
Published in:    IEC 60335-1, ed. 5.0 (2010-05)
(For information only, not the topic of this discussion)
insulation
that part of an electrotechnical product which separates the conducting parts at different electrical potentials
Published in:    IEC 60664-2-1, ed. 2.0 (2011-01)
insulation coordination
mutual correlation of insulation characteristics of electrical equipment taking into account the expected micro-environment and other influencing stresses
Published in:    IEC 60664-2-1, ed. 2.0 (2011-01)
impulse withstand voltage
highest peak value of impulse voltage of prescribed form and polarity which does not cause breakdown of insulation under specified conditions
Published in:    IEC 60664-2-1, ed. 2.0 (2011-01)
overvoltage
any voltage having a peak value exceeding the corresponding peak value of maximum steady-state voltage at normal operating conditions
Published in:    IEC 60664-2-1, ed. 2.0 (2011-01)    
lightning overvoltage
transient overvoltage at any point of the system due to a specific lightning discharge
Published in:    IEC 60664-1, ed. 2.0 (2007-04)
withstand voltage
voltage to be applied to a specimen under prescribed test conditions which does not cause breakdown and/or flashover of a satisfactory specimen
Published in:    IEC 60664-2-1, ed. 2.0 (2011-01)
The isolating transformers in question can be divided into three groups; low frequency power, high frequency power (switching mode power supplies) and signal (e.g. Ethernet data).
That’s part of the problem. Power engineers will tend to only think about low frequency power frequency transformers. Telecommunication engineers will be familiar with all three uses (low frequency power, high frequency power and signal). If the two engineering disciplines don’t think alike, there is potential for misunderstanding. Power engineers will know that the low frequency transformer spectrum is below the frequency spectrum of lightning. Telecommunications engineers will appreciate that for the other two types of transformer, the high frequency power or signal have spectrums may overlap or be above the lightning spectrum.
This disparity of thinking was very evident when the topic was first promoted for standardisation several years ago. At the second coming the US National Committee simply refused to vote!
December 2012 had 5 Replies & 4 Comments

    1
Michael Maytum 3 months ago

  Is an isolating transformer a surge protective component SPC? Part 2
  As the tailpiece to Part 1 indicated, the recent hoo-hah has started be because IEC SC 37B (Specific components for surge arresters and surge protective devices) has formed Working Group 3 as a result of a World wide National Committee ballot. The remit of WG 3 is to produce documents on the performance requirements, test methods and application principles for Lightning Isolation Transformers (LITs).
  The work was instigated by the Japanese National Committee mainly as a result of home networking equipment failures, see http://www.itu.int/ITU-T/worksem/techsessions/com05/110429/index.html. It was found that surges on the AC supplies (like the US there are usually two 120 V AC feeds in a house) were breaking down power supply and Ethernet transformer insulation barriers and causing equipment failure. Power substations have the occurrence of large differential voltages too.
  As insulation barriers are being tested the classic insulation test waveshape of 1.2/50 is preferable. The peak voltage test levels required are likely to be in the 1 kV to 10 kV range.
  As a quick proof that isolating transformers are used in surge protector’s view http://www.pro-tek5.com/pdf/ptg-5t.pdf. This is a data sheet for an Inter-Building Lightning Strike Protector and the circuit diagrams show it uses two types of SPC; a 5 kV rated isolating transformer and a voltage limiter across the output winding. Here’s the important point, the transformer isolates longitudinal/common mode surges and the voltage limiter is there for the transverse/metallic/differential surges.
  Unfortunately some low-frequency power engineers miss the point that the transformer is there for longitudinal/common mode protection and start talking about transverse/metallic/differential surges. The results of transverse/metallic/differential surges depend on the type of isolating transformer (low frequency power, high frequency power or signal).
  The low-frequency power engineers, thinking of their transformers say the isolating transformer doesn’t offer any surge protection in this case. The high inductance of the winding prevents any current diversion. Quite correct, but apply a longitudinal surge on L and N and it will stop the transfer of surge voltage.
  It’s a strange thing that AC power surge testing is mostly done on a single conductor, yet the Japanese work indicated that many of the surges were on both live and neutral.
  Perversely IEC TC 81 carried this concept over to telecommunications testing; failing to explain how one wire of a tightly coupled twisted pair could have a surge current in it yet there was no coupling to the other wire, thereby breaking the laws of physics.
  In summary, an isolating transformer provides longitudinal/common mode surge protection; a voltage or current limiter is needed for transverse/metallic/differential protection.

    2
Michael Maytum 3 months ago

  Is an isolating transformer a surge protective component SPC? Part 3
  Some people discussing this topic make a great play of definitions based on those forged many years ago and hence don’t comprehend technology evolution or new applications or new technologies. Look at this selection.
  surge protective device, SPD
   
  device intended to limit transient overvoltages and divert surge currentsIt contains at least one non-linear component.TC/SC: 88, Published in: IEC 61400-24, ed. 1.0 (2010-06)
  device that contains at least one nonlinear component that is intended to limit surge voltages and divert surge currentsNOTE An SPD is a complete assembly, having appropriate connecting means.TC/SC: 37A WG5, Published in: IEC 61643-11, ed. 1.0 (2011-03)
  device intended to limit transient overvoltages and divert surge currents; contains at least one non-linear componentTC/SC: 81, Published in: IEC 62305-4, ed. 2.0 (2010-12)
  device that restricts the voltage of a designated port or ports, caused by a surge, when it exceeds a predetermined level
  NOTE 1 Secondary functions may be incorporated, such as a current-limiting to restrict a terminal current.
  NOTE 2 Typically the protective circuit has at least one non-linear voltage-limiting surge protective component.
  NOTE 3 An SPD is a complete assembly, having terminals to connect to the circuit conductors.TC/SC: 37A WG4, Published in: IEC 61643-21, ed. 1.0, amd. 1 (2008-04)
  Only the last definition, created by SC 37A WG4 communications engineers, not power engineers, is the most complete, because it informatively mentions secondary functions and the use of a non-linear voltage-limiting surge protective component as only “typical”.
  Why only “typical”? One reason is coaxial high frequency communications systems operating at hundreds of Mhz. Here the communications spectrum is above the lightning spectrum and the lightning spectrum can be filtered out by a stub, see http://www.timesmicrowave.com/products/protect/downloads/STRN.pdf. This is a linear device having very low PIM (Passive Intermodulation) distortion of <-160 dBc. A design using a non-linear shunt GDT component might typically have a PIM distortion of -110dBc, see http://www.timesmicrowave.com/downloads/products/LP-GTV.pdf. The advantage of the GDT based surge protector is it can be applied in situations where the communications spectrum overlaps the lightning spectrum.
  The failing of the first three definitions is they don’t concentrate only on what the SPD does, but dictate how it operates. The fixation with voltage limiter operation means that the definition term should be “Voltage-Limiting type SPD” not the shortened “SPD”.
  This is important because current-limiting SPDs are now possible using electronic current limiting (ECL) components. The two limiting types can be described as:
  Voltage-Limiting type SPD operation
  Overvoltages that exceed a predetermined voltage threshold are limited and the consequential current from the overvoltage conducted by the shunt voltage limiting element.
  The dual of which is:
  Current-Limiting type SPD operation
  Overcurrents that exceed a predetermined current threshold are limited and the consequential voltage from the overcurrent blocked by the series current limiting element.
  Of course none of this “Voltage-limiting” or “Current-limiting” qualification would be necessary if the guys writing the definitions stuck to describing what an SPD does.
  In conclusion, most SPD definitions are flawed for generic use. With that bombshell I finish this contribution.

Michael Maytum 3 months ago
    Is an isolating transformer a surge protective component SPC? Part 4
    The previous discussion showed how an isolation transformer can be qualified as a surge protective component. The discussion also showed how important it is to describe what the thing does and avoid mandating how it does it. Mandating how it does it locks you into specific technologies.
    From our knowledge of different types of surge protective component a general definition can be put together avoiding the cited SPD definition mistakes.
    Where is an SPC used? It’s not just in SPDs but in equipment ports as well.
    The component responds to overvoltages or overcurrents or both to reduce their onward propagation levels.
    The component is unlikely to operate at when the normal system voltage or current is exceeded (onset of the overvoltage or overcurrent) but at a slightly higher level. In ITU-T speak it mitigates (make (something bad) less severe, serious, or painful) the overstress. The overstress is not removed but reduced to a predetermined level set by the technology.
    surge protective component SPC
    component designed into the circuit of a device or equipment port, for the purpose of mitigating the onward propagation of overvoltages or overcurrents or both.
    NOTE 1 The selected component should not significantly degrade the normal system operation.

Michael Maytum 3 months ago
    The IEC has descended into are rather pointless debate that just started off on isolation transformers. One guy is demanding that the Power SPD definition be enforced on the Telecom engineers as their Telecom definition. My reply was as follows:
    You are running several crusades at present. Here I am replying to your shock-horror that IEC SC37A WG4 has a different definition of an SPD compared to IEC SC37A WG5.
    First point is WG4 – Telecom - and WG5 – Power - are different engineering disciplines. Both use metallic conductors and are afflicted lightning disturbances. But that’s about it.
    The metallic conductors used in Telecom engineering are typically balanced-signal twisted-pair conductors, unbalanced-signal coaxial cables and unbalanced-balance conductor pairs. The cables may or may not be shielded (screened).
    The power boiler-plate of 1000 V rms or 1500 V DC does represent Telecom signal levels. Telecom powering is typically a maximum of 400 V DC and the maximum signal found is cadenced 150 V rms for POTS line ringing.
    Telecom engineering is constantly evolving in terms of equipment and techniques. Had we been told 15 years ago that the voice band (6 kHz) telephone cables coming to our houses where destined to be used to supply broadband data anywhere from 30 MB/s to 2 MB/s it would seem like a fairy story. Home networking is now common-place and installed by the householder.
    The diversity within Telecom engineering means that a wide range of surge mitigation techniques are used, not just the ones used in Power engineering. As a result the Power engineering SPD definition can be viewed as a subset of the Telecom SPD definition.
    The surge mitigation techniques available in Telecom are:
    Non-linear
    Voltage limiting
    Current limiting
    Linear
    Insulation barrier i.e. isolating transformers
    Frequency filtering e.g. quarter-wave stubs for coaxial cables
    It makes no sense trying to force a totally inappropriate definition of a term on a radically different engineering discipline.
    Where harmonisation isn’t sensible then co-existence is the correct answer. There needs be the co-existence of a power SPD definition and a Telecom SPD definition – each to their own.
    Regards
    Mick Maytum

    3
Raymond Hill 3 months ago

  Very informative discussion.  At first glance, I was of the opinion that when applying the present definition of an SPD which states that there must be at least one non-linear device, that a transformer would not qualify.  However, having an electric utility background, I know that "neutralizing transformers" have been used in electric utility substations on communications circuits successfully for many decades.  I believe the definition of an SPD should be modified in order to remove the "non-linear" reference.
  I have to state, however, that a common-mode impulse impinging on the input of a transformer, unless the primary and secondary coils have proper electrostatic shielding, will capacitivley couple a surge into the output circuit.  Depending on the physical layout/geometry of the transformer and the output circuit and load, the surge coupling to the output can have common-mode and differential-mode characteristics.

Michael Maytum 3 months ago
    Ray
    The use of an electrostatic shield is already comprehended in the draft IEC SC 37B WG3 document (37B/108/NP -  NEW WORK ITEM PROPOSAL - 2012-04-13). Here are some definitions on the shield and transformer.
    3.2.1 Lightning Isolation Transformer, LIT
    Isolation transformer  which has  high impulse withstanding voltage with/without  shielding between input and output  windings.   
    3.2.2  Electrostatic Shielding Plate, ESP
    The conductive shielding material between  the windings of the LIT which is used to ground  the
    LIT and provides a electrostatic shield between the windings
    3.2.4 Impulse Withstand Voltage
    The mutual impulse withstand voltage between  the input winding, the output winding and the
    ground of the transformer.
    Mick note: ground would be the ESP connection
    3.2.5  Surge Voltage Transfer Ratio (common mode)
    The ratio  (VOUT/VIN) of  the voltage between the output winding and ground  (VOUT) and the
    impulse voltage between the input winding and the ground (VIN) when a 1.2/50 impulse voltage
    waveform is applied
    Mick note: marketing people would want to turn this number upside-down so it was big. Me for Telecom, I'd like to see the equivalent dB value e.g. -80 dB.
    Regards
    Mick

Michael Maytum 3 months ago
    “I believe the definition of an SPD should be modified in order to remove the "non-linear" reference.”
    Ray,
        We have two very different engineering disciplines here; Power and Telecom. In spite of the noted flaws with the present power engineering definition of an SPD, it has served the power community well because of the dominance of the voltage limiting as a solution. The large number of telecom system variants is a reason a telecom SPD needs to be described by its function rather than tying it to a solution.
        We need co-existence rather harmonisation of definitions here. Thus I favour the creation of a generic or telecom SPD definition and leave the power guys with what they are used to.
        The IEC 61643-21 definition is a good start, but because of the different interpretations of “surge” I would substitute “overvoltages or overcurrents or both”. This would avoid the problem that some power guys have by telecom regarding AC power intrusion as an external electrical event that causes surges.
        Telecom uses non-linear and linear technologies as appropriate for protection solutions:
    Non-linear
        Voltage limiting
        Current limiting
    Linear
        Insulation barrier i.e. isolating transformers
        Frequency filtering e.g. quarter-wave stubs for coaxial cables

    Mick

    4
Michael Maytum 2 months ago

  Isolating and neutralizing transformers
  Neutralizing transformers and isolating transformers have different ways of mitigating surges. The isolating transformer blocks the onward propagation of a common-mode surge on a communication line pair with an insulation barrier. The neutralising transformer bucks out ground potential rise differences between separated grounds on the communication line. The neutralising transformer can also buck out magnetically induced voltages.
  A phase-to-ground fault on a power transmission system can result in a ground potential rise (GPR) or increase in ground potential at the power station with respect to remote earth. The GPR can be disruptive and damaging to wire-line communications facilities entering the power station unless mitigative devices are employed. A neutralizing transformer bucks out this potential difference in communications lines.  
  A neutralizing transformer consists of closely coupled windings on a ferromagnetic core. The transformer primary winding is connected between power station and remote grounds. A secondary winding is placed in series with each conductor of the communications wire line entering the power station. The GPR voltage difference is applied to the transformer primary. The secondary windings, in series with the communications lines, are connected in anti-phase so their transformed voltage opposes the GPR voltage difference. Ideally the communication line transformed voltage and the GPR voltage difference cancel out, neutralising the the GPR voltage difference.

    5
Tatsuhiko Katayama 3 weeks ago

  Is an isolating transformer a surge protective component SPC?
   
  Isolating transformers have been used as surge protective equipments (or devices).  Though these transformers are not SPD defined as containing at least one non-linear component, they could be regarded as a part of surge protective equipments (or SPDs?).
   
  Isolating transformers are usually used inside of terminal equipments, if necessary, in telecommunication equipments, whose designers know the required performances including withstand voltages.  It seems there would be no strong demand in the market to standardize these isolating transformers.
   
  Isolating transformers are often used for low voltage power distribution entrance with/without SPDs.  Their impulse withstand voltage of 1.2/50, 30 kV are seemed to be a “de facto” standard for these isolating transformers used in Japan, regardless of with or without SPDs.
  There seems to be some demand in the market to standardize these isolating transformers, because:
  ・They are often used stand-alone, i.e. without SPDs, as “Lightning Withstand Transformer”.
  ・Those SPDs, which are used with “Lightning Withstand Transformer”, are often tested only in Class II. (If they are to withstand “Lightning” effects, i.e. remain protective against part of lightning current, they should be “protected” by Class I tested SPDs.)
  ・The Voltage Protection Level Up of these SPDs used for “Lightning Withstand Transformer” are often less than 1.5 kV (in case Uc = 100 V), though the impulse withstand voltage of the transformers is 1.2/50, 30 kV.
   
  The needs of standardization of isolating transformers for low voltage power distribution system would be to rationalize these mismatches in the specification and to clarify the parameters to be considered, so that users could select suitable ones correctly.

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Isolating Transformers

 

A separate thread on isolating transformers has been running in the ITU-T. Here is the thread to date
 
Michael John Maytum 11 Dec 2012 08:33 AM
 
Isolating transformers
 
There has been a great deal of discussion in the IEC and IEEE about the validity of an isolating transformer to be classed as a surge protective component or a surge protection device. This thread is to determine the Q2/5 (Protective components and assemblies) position on the isolating transformers.
 
Albert Martin 11 Dec 2012 09:36 PM
What definition are you using for a surge protective component and a surge protective device?
 
Michael John Maytum 12 Dec 2012 04:03 PM 
Q. “What definition are you using for a surge protective component and a surge protective device?”
 
A. Unfortunately the published definitions for these components and devices are mostly unhelpful as they are inadequate for generic use. They fail in two main areas;
    1. The people writing the definitions are only considering a specific event or application or both and
     2. The definition describes a technology operation and not the function.
 
Thus it becomes necessary to develop a generic definition for the phrases from first principles - a word at a time.
 
Physical items
Many people wrongly call a component a device. Reference to the IEC IEV (dictionary) would help such people.
 
IEV 151-11-20
device    
material element or assembly of such elements intended to perform a required function
 
IEV 151-11-21
component
constituent part of a device which cannot be physically divided into smaller parts without losing its particular function
 
IEV 151-11-22
apparatus
device or assembly of devices which can be used as an independent unit for specific functions
 
IEV 151-11-25
equipment
single apparatus or set of devices or apparatuses, or the set of main devices of an installation, or all devices necessary to perform a specific task
 
That leaves “surge” and “protective” to be discussed.
 
Albert Martin 12 Dec 2012 04:46 PM 
I think we need a definition of "isolating transformer" as well. Once we have the definitions we should be able to see if "isolation transformer" meets either the definition of a surge protective component or a surge protective device
 
Michael John Maytum 13 Dec 2012 10:06 AM 
Al,
Standardised definitions for an isolating transformer are at
 
We have started to define the SPD and SPC generic functions. This needs to be done first.
 
Did you miss my point that published definitions for these components and devices are mostly unhelpful as they are inadequate for generic use.
 
When we have generic functional SPD and SPC definitions we can test against the functional definition of specific linear transformers (isolating, neutralising and EMC chokes). Note there are three classes of linear transformer that can be used for common mode surge attenuation. Each class will need it's own functional definition as regards to surge performance.
 
Michael John Maytum 13 Dec 2012 09:48 AM
What is a “surge”
    Most readers will have a definition for a surge in their mind. How do the standards organisations define it?
 
IEC
non-periodic and relatively short positive or negative (or both) variable (voltage or current) between two steady states
IEC 60571, ed. 3.0 (2012-09)
transient wave of electrical current, voltage, or power propagating along a line or a circuit and characterized by a rapid increase followed by a slower decrease
IEC 61000-4-5, ed. 2.0 (2005-11)
transient created by LEMP (lightning electromagnetic impulse) that appears as an overvoltage and/or an overcurrent
IEC 62305-1, ed. 2.0 (2010-12)
 
ITU
 
Temporary excessive voltage or current, or both, coupled on a telecommunication line from an external electrical source.
ITU-T K.46 (05/2012), ITU-T K.89 (05/2012)
Overvoltage and/or overcurrent of a transient nature that may be imposed on a conductor as a consequence of an electromagnetic disturbance.
K.46 (03)
Temporary excessive voltage or current, or both, coupled on a telecommunication line, from an external electrical source.
K.65 (04)
 
IEEE
 
A transient voltage or current, which usually rises rapidly to a peak value and then falls more slowly to zero, occurring in electrical equipment or networks in service.
IEEE Std 4-1995
A transient wave of current, potential, or power in an electric circuit.
IEEE Std C62.11™-2005
A transient wave of voltage or current. The duration of a surge is not tightly specified, but it is usually less than a few milliseconds.
IEEE Std C62.34-1996
An impulse, occurring in electrical equipment or networks in service.
IEEE Std 1783™-2009
 
The above list is a mix of opinions.
The ITU (Telecommunications) only classifies an electrical disturbance as a surge when the amplitude is an overvoltage or overcurrent or both. The surge is not defined in terms of waveshape or duration.
The IEEE is more pedantic on the waveshape and timings without defining amplitude. It completely misses the situation of a truncated surge which has a relatively slow rise and rapid decay.
The IEC only considers short term disturbances. One case considers a disturbance that is only an overvoltage or overcurrent or both and due to lightning.
 
The problem with the phases “overvoltage” and “overcurrent” is they are system dependent. More correctly for non-linear SPDs these phases relate to voltages or currents that exceeds the SPD limiting levels.
 
A working generic definition for this discussion will be taken as:
 
surge
temporary disturbance on the conductors of an electrical service caused by an electrical event.
 
This is a superset of the standards organisations definitions.
 
 
Michael John Maytum 15 Dec 2012 11:36 AM 
What is a “protective”?
    Defining the middle word of SPC and SPD is the most difficult. The Shorter Oxford Dictionary has:
Protective: having the quality or character of protecting; tending to protect; defensive; preservative.
 
Because of history we are going to be stuck with using the word “protective”. “Protective” is a system, not a component or device, property that depends on not only the component or device functions but also the surge and the downstream circuit withstand. To say the protective function protects in all circumstances is generally not true. In the test laboratory with a defined surge and with a defined protected item withstand one can say protection is achieved. By itself a “protective” function can be said to mitigate (reduce) a surge. Even then some load condition is implied such as an open-circuit for a voltage limiting protective function or a short-circuit for current limiting protective function.
 
For a standalone surge protective function situation, the word “mitigation” (the action of reducing the severity, seriousness, or painfulness of something) is a more appropriate word than “protective”. Mitigation can achieved by either surge restriction (e.g. clamping) or surge reducing (e.g switching). To cover these situations a general phrase "reduction or restriction" will be used
 
Michael John Maytum 15 Dec 2012 01:11 PM
What does an SPD and an SPC do?
 
The key action word is protective. A summary of the “protective” word discussion for an SPD could be:
 
protective (SPD): surge reduction or restriction at the device protected terminals.
 
SPCs are a little trickier to define as the component may represent all or part of the surge protection circuit.
 
protective (SPC): component used to implement a surge reduction or restriction circuit.
 
To these “protective” definitions we need to add an informative note on protective modes
 
NOTE 1 – The surge reduction or restriction operating modes may be common-mode or differential-mode or both.
 
Using these “protective” definitions existing SPD and SPC definitions can be reviewed.
 
Michael John Maytum 16 Dec 2012 10:44 AM
Review of SPD definitions from power engineering
 
This section looks at how the power engineering community define surge protective devices. Four examples are given
 
surge protective device, power SPD
device that contains at least one nonlinear component that is intended to limit surge voltages and divert surge currents
NOTE An SPD is a complete assembly, having appropriate connecting means.
IEC 61643-11, ed. 1.0 (2011-03)
 
surge protective device, power SPD
device intended to limit transient overvoltages and divert surge currents It contains at least one non-linear component.
IEC 61400-24, ed. 1.0 (2010-06)
 
surge protective device, power SPD
device intended to limit transient overvoltages and divert surge currents; contains at least one non-linear component
IEC 62305-4, ed. 2.0 (2010-12)
 
surge-protective device, power SPD
A device that is intended to limit transient overvoltages and divert surge current, or both. It contains at least one nonlinear component.
IEEE Std C62.34-1996, IEEE Std C62.62™-2010
 
     •All four definitions state a non-linear component must be used.
     •All four definitions state voltages are limited (restricted).
     •All four definitions state that surge currents are diverted
     •Three definitions state voltage is limited and current diverted.
    (IEEE Std C62.34 has the peculiar construction “limit transient overvoltages and divert surge current, or both”! May be the "and" should be an "or".)
     •Three definitions state it is only overvoltages that are acted on
 
These examples show the mind set of the power engineering guys – closed to anything that isn’t non-linear and “limits voltages & diverts surge currents”. This explains why they can’t understand how an isolating transformer (a linear transformer) can be classed as a surge protective component. Yet bizarrely they are happily to accept a neutralising transformer (a linear transformer) as giving surge protection to sub-station communications lines!
 
The bottom line is that these power SPD definitions are not generic, but highly specific to practice in power engineering.
 
They are all subsets of “surge mitigation at the device protected terminals”.
 
Michael John Maytum 16 Dec 2012 11:42 AM 
Review of SPD definitions from telecommunications engineering
 
This section looks at how the telecommunications community define surge protective devices. Eight examples are given
 
surge protective device, Telecommunications SPD
device that restricts the voltage of a designated port or ports, caused by a surge, when it exceeds a predetermined level
NOTE 1 Secondary functions may be incorporated, such as a current-limiting to restrict a terminal current.
NOTE 2 Typically the protective circuit has at least one non-linear voltage-limiting surge protective component.
NOTE 3 An SPD is a complete assembly, having terminals to connect to the circuit conductors.
IEC 61643-21, ed. 1.0, amd. 1 (2008-04)
 
surge protective device, Telecommunications SPD
Device that restricts the voltage of a designated port or ports, caused by a surge, when it exceeds a predetermined level.
1) Secondary functions may be incorporated, such as a current-limiting device to restrict a terminal current.
2) Typically, the protective circuit has at least one non-linear voltage-limiting surge protective component.
3) An SPD is a combination of a protection circuit and holder.
ITU-T K.89 (12), K.65 (04), K.72 (11)
 
surge protector, Telecommunications SP
A protective device, consisting of one or more surge-protective device components, a mounting assembly, optional fuses and short-circuiting devices, etc, which is used for limiting surge voltages on low-voltage (≤1000 V rms or ≤1200 V dc) electrical and electronic equipment or circuits.
IEEE Std C62.31™-2006
 
surge protector, Telecommunications SP
An assembly of protective devices consisting of one or more series, parallel, or any combination of elements used to limit surge voltages, currents, or both to a specified level. Syn: protector.
IEEE Std C62.36-2000
 
surge protective device, Telecommunications SPD
A device that is intended to limit overvoltages and divert overcurrents across its terminals. The gas discharge tube (GDT) is a common type of SPD.
ITU-T K.46 (03)
 
surge protective device, Telecommunications SPD
A device that is intended to mitigate surge overvoltages and overcurrents of limited durations. It may consist of a single component or have a more complex design, where several functions are integrated. It contains at least one non-linear component.
ITU-T K.46 (12)
 
surge protective device, Telecommunications SPD
A device that is intended to limit transient overvoltages and divert surge currents. It contains at least one non-linear component.
ITU-T K.25 (00), K.39 (96),K.40 (96)
 
surge protective device, Telecommunications SPD
A device that is intended to mitigate surge overvoltages and overcurrents of limited durations. It may consist of a single component or have a more complex design, where several functions are integrated. It contains at least one non-linear component.
ITU-T K.36 (96), K.44 (03)
 
    • six definitions state voltages are limited (restricted)
    • five definitions mention just surge or surge voltages and currents
    • five definitions mention surge currents
    • four definitions state it is only overvoltages that are acted on
    • three definitions state a non-linear component must be used
    • two definitions state that voltage is limited and current diverted
    • two definitions use the word “mitigate” rather than describe the action
 
This is a broader mind set compared to power engineering. Less than half of the definitions stipulate a non-linear component. Less than half of the definitions stipulate a current diversion. Half of the definitions state it is only overvoltages that are acted on. More than half of the definitions simply reference surge or surge voltages and currents. More than half of the definitions mention surge currents. Most definitions focus on voltage (mitigation).
 
One could construct a generic definition using parts of K.36/44, C62.36 and K.89/65/72.
 
“device that is intended to mitigate surge voltages or currents or both of limited durations at a designated port or ports”
 
This is in harmony with
“surge mitigation at the device protected terminals”
 
Michael John Maytum 16 Dec 2012 01:06 PM
Surge mitigation
 
Those who have followed this thread thus far should be aware the prime requirement for an SPD or SPC is that it mitigates the surge. The mitigation function does not have to be non-linear, it can be linear.
On this basis, the question that needs to be answered is “does an isolating transformer provide surge mitigation?”.
 
An isolating transformer is defined as:
 
isolating transformer
transformer with protective separation between the input and output windings
IEC 60065, ed. 7.0 (2001-12)
 
Inherently the isolating transformer is an in-line two or more port component, where surges may occur
    • differential-mode at a port or
    • common-mode from the port to the local earthing system.
 
Differential-mode surge at a port
If the transformer maintains its linearity there will be no mitigation of the surge. The non-surged port will receive the surge essentially unabated apart from whatever the port-to-port turns ratio scales it to.
 
Common-mode surge from the port to the local earthing system.
If the transformer insulation withstands the surge voltage, the common-mode surge will be mitigated at the non-surged port. Any inter-winding capacitance will give a capacitive feed-through of the higher frequency surge components.
 
Summary
The isolation transformer is an SPC as it provides surge mitigation for common-mode surges; however it does not provide surge mitigation for differential-mode surges.
 
Albert Martin 17 Dec 2012 02:25 AM 
So an isolation transformer can be defined as an SPC that mitigates common-mode surges? [perhaps with a note that it doesn't mitigate differential-mode surges]
 
Michael John Maytum 17 Dec 2012 10:54 AM
Al,
    You should be writing this thread – you’re always two steps ahead of me!
 
Yes, we need to state the mitigation capabilities and limitations of the isolation transformer technique. That will be in the definition for the Lightning Isolation Transformer, not for an isolation transformer, whose adequate definition already exists.
 
To round off surge mitigation transformers I will next be covering the neutralising and common mode choke types.
 
Albert Martin 17 Dec 2012 03:33 PM 
Is "Lightning Isolation Transformer" the right term, or should it be "Common Mode Isolation Transformer"? Because it protects not only against lightning, but any common mode disturbance.
 
Michael John Maytum 18 Dec 2012 10:18 AM 
neutralising transformers and common-mode chokes
 
This part discusses neutralising transformers and common-mode chokes
 
Neutralising transformers
    These are typically used in power substations to buck out the ground potential rise differences between separated grounds on the communication line. The neutralising transformer can also buck out magnetically induced voltages on the communication line.
A phase-to-ground fault on a power transmission system can result in a ground potential rise (GPR) or increase in ground potential at the power station with respect to remote earth. The GPR can be disruptive and damaging to wire-line communications facilities entering the power station unless mitigative devices are employed. A neutralizing transformer bucks out this potential difference in communications lines.
 
A neutralizing transformer consists of closely coupled windings on a ferromagnetic core. The transformer primary winding is connected between power station and remote grounds. A secondary winding is placed in series with each conductor of the communications wire line pair entering the power station. The GPR voltage difference is applied to the transformer primary. The secondary windings, in series with the communications lines, are connected in anti-phase so their transformed voltage opposes the GPR voltage difference. Ideally the communication line transformed voltage and the GPR voltage difference cancel out, neutralising the GPR voltage difference.
The only definition found was the following and it reads like a book.
 
neutralizing transformers and reactors: Devices that introduce a voltage into a circuit pair to oppose an unwanted voltage. These devices neutralize extraneous longitudinal voltages resulting from ground potential rise, or longitudinal induction, or both, while simultaneously allowing ac and dc metallic signals to pass. These transformers or reactors are primarily used to protect telecommunication or control circuits at supply locations or along routes where exposure to power line induction is a problem, or both.
IEEE Std 487™-2007
 
Common-mode chokes or common-mode filters
    These are two winding transformers. For a twisted pair each winding is placed in series with a conductor and poled to give a transformer action that presents high impedance to common-mode signals and low impedance to the wanted differential signal. Coaxial cables can be wrapped round a magnetic core to give the same effect.
 
I couldn’t find a definition for either “common-mode choke” or “common-mode filter”. A definition might take the form:
 
common-mode choke/filter: series in-line transformer used to mitigate common-mode current flow without affecting differential current flow.
 
One could change current flow to voltage, series to shunt and arrive at a LIT definition viz:
 
lightning isolation transformer: shunt in-line transformer used to mitigate common-mode voltage without affecting differential voltage.
Just a thought.
 
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Isolating transformer - impulse withstand voltage test values

 TC 109 insulation impulse voltage test values for Overvoltage category IV are:

0.8 kV, 1.5 kV, 2.5 kV, 4 kV, 6 kV, 8 kV, 12 kV

The 1.5 kV, 4 kV and 6 kV values match ITU-T test values. The 0.8 kV value is close to the ITU-T 1 kV test level. The 2.5 kV is close to the IEEE 802.3 value of 2.4 kV. Modelling and extending these TC 64 values gives a preferred  value series of:
1.5 kV, 2.5 kV, 4 kV, 6 kV, 8 kV, 12 kV, 15 kV, 25 kV, 40 kV, 60 kV, 80 kV and 120 kV

We should consider harmonising with the IEC on test voltage values.

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Isolating transformer core saturation

After thinking about signal transformer core saturation truncating the surge, I think the most useful thing for users is to define transformer saturation volt-seconds for a given winding (integral of the applied winding voltage and time before magnetic core saturation) and a test for determining the saturation volt-seconds. An application guide can define the significance of the saturation volt-seconds on the transformed waveform.
And we ought to be thinking about defining surge mitigation (reduction of the peak value of a specified surge event by linear or non-linear means).

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Isolating transformer - Inter-winding capacitance

The transformer inter-winding capacitance provides another route for fast rising common-mode surges to transfer from the primary to secondary. When the secondary is balanced, the transferred surge appears as a common mode surge on both ends of the secondary winding (assuming the inter-winding ends are reasonably matched on capacitance). When the secondary is unbalanced, the transferred surge appears as a differential mode surge on the unearthed end of the secondary winding (the earthed end will transfer its capacitive current to earth). Any transformer screen will reduce the inter-winding capacitance and the level of transferred current. The attachment shows the balanced winding and unbalanced winding capacitance measurement circuits.

 

 

 

 

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IEEE Standard

Seems to me we now have enough material for WG 3.6.2 to formulate a document named "IEEE Standard for Test Methods and Preferred Values for the Surge Parameters of Telecommunication Signal Isolating Transformers".

Definitions have been covered and the key surge parameters for isolation transformers defined as:

  1.      Impulse withstand voltage (Rating)
  2.     Inter-winding capacitance (Characteristic)
  3.     Core saturation volt-second integral (Characteristic)
  4.     Insulation resistance (for rating verification) (Characteristic)
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Rated impulse voltage and impulse withstand voltage

Rated impulse voltage and impulse withstand voltage

The impulse testing of insulation is different to normal impulse (surge) testing.

Reference documents for insulation rating and testing are:

IEC 60664-1, Ed. 2.0: Insulation coordination for equipment within low-voltage systems - Part 1: Principles, requirements and tests

IEC/TR 60664-2-1 Ed. 2.0: Insulation coordination for equipment within low-voltage systems - Part 2-1: Application guide -Explanation of the application of the IEC 60664 series, dimensioning examples and dielectric testing

Firstly there is the insulation rating and the test voltage used to verify that rating.

rated impulse voltage

impulse withstand voltage value assigned by the manufacturer to the equipment or to a part of it, characterizing the specified withstand capability of its insulation against transient overvoltages

impulse withstand voltage

highest peak value of impulse voltage of prescribed form and polarity which does not cause breakdown of insulation under specified conditions

These two definitions look similar, but the rated impulse voltage is the component specified value and the impulse withstand voltage is the voltage used to verify the rated impulse voltage. It makes sense that the test voltage should not be less than the rated value. In fact, at rated voltages of 4 kV and above the nominal test voltage is some 20 % higher.

Here are the preferred rating values and their sea level nominal test voltages shown in parenthesis.

 

0.33 kV (0.357 kV)

0.5 kV (0.541 kV)

0.8 kV (0.934 kV)

1.5 kV (1.751 kV)

2.5 kV (2.920 kV)

4.0 kV (4.923 kV)

6.0 kV (7.385 kV)

8.0 kV (9.847 kV)

12.0 kV (14.770 kV)

 This is the first unique difference – you verify at a higher level voltage than the rating.

 Second a 1.2/50 voltage generator is used; don’t confuse this with the 1.2/50-8/20 combination wave generator. The insulation test generator has a peak voltage tolerance of ±5 %; the combination wave generator has a peak voltage tolerance of ±10 %. The 1.2/50 voltage generator doesn’t have a specified current waveform because good insulation doesn’t breakdown - right. If the insulation does breakdown and fries then that’s bad insulation anyway. The withstand test shall be conducted for a minimum of three impulses of each polarity with an interval of at least 1 s between pulses

References:

IEEE Std 4TM-1995, IEEE Standard Techniques for High Voltage Testing

IEC 60060-1 ed. 3.0 (2010-09) High-voltage test techniques –Part 1: General definitions and test requirements

 This is the second unique difference – you test with ±5 % amplitude tolerance 1.2/50 voltage generator.

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Ronald B. Standler, Ph.D.

 

Opponents to calling an isolating transformer a surge protective component claim that an SPD must limit voltage and divert current because Ronald Standler helped craft that definition.

Certainly Mr Standler is the most brilliant person I ever met and was years ahead of the majority of protection engineers. He published the epic book “Protection of Electronic Circuits from Overvoltages”. This book has been one of my major references over the years and is now available in electronic format. I recommend this book for any low-voltage broad-based protection engineer. Hint to a loved one that it would make a nice present or, failing that, treat yourself!

The point of mentioning the book is that it shows Mr Standler was not hung up over voltage limiting SPDs as the only means of protection. The book – concerned with “Protection” - has chapters on filters, EMC chokes and – wait for it – isolating transformers! These to Mr Standler are valid protection techniques.

People who insist that protection can only be achieved by voltage limiting and current diversion and cite Mr Standler, haven’t read or understood Mr Standler’s viewpoint on protection.

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PC62.69 - Surge Parameters of Isolating Transformers

An outcome of this thread is the approval of PC62.69

PROJECT NUMBER: PC62.69
Group: WG3.6.2
Project Type: New
Title: Standard for the Surge Parameters of Isolating Transformers Used in Networking Devices and Equipment
Scope: This standard sets terms, test methods, test circuits, measurement procedures and preferred result values for the surge parameters of isolating transformers used in networking devices and equipment. Three types of isolating transformer are considered; mains low frequency power, high frequency power (switching mode power supplies) and signal (e.g. Ethernet data). The surge parameters of the isolating transformer insulation barrier covered by this standard are:
- Rated impulse voltage
- Input winding to output winding capacitance
- Insulation resistance
Additional parameters for signal isolating transformers are:
- Core saturation voltage-time product
- Rated input winding rms current for a given temperature rise
This standard does not cover the transformer parameters required to ensure appropriate operation on the service e.g. signal transformer return loss.
PAR Expiration: 12/31/2017
URL: https://development.standards.ieee.org/P831400033/par

 

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IEEE Std 802.3T

There has been a great deal of confusion and discussion on the testing of Ethernet ports to IEEE Std 802.3T. A document posted on this site, Click here to download it, tries to identify what causes this Ethernet port.debate. It is hoped to develop a sensible test approach from this document, which serves both safety and reliability needs.

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LV power isolating transformer standardization

The IEC subcommittee SC 37B in working group 3 has two projects on LV isolating transformers. The proposed document titles are:

  • IEC 61643-361: Low-voltage surge protective components - Part 361: Lightning isolation transformers (LIT) connected to low-voltage distribution system - Requirements and test methods
  • IEC 61643-362: Low-voltage surge protective components - Part 362: Lightning isolation transformers (LIT) connected to low-voltage distribution system - Selection and application principles

Unfortunately there is a "turf" issue with another IEC committee, which will hold up the development until it is resolved.

The companion documents on signal isolating transformers are proceeding quite rapidly. These are:

  • IEC 61643-351 Low-voltage surge protective components - Part 351: Lightning isolation transformers connected to telecommunications and signalling networks-Requirements and test methods

  • IEC 61643-352 Low-voltage surge protective components - Part 352: Lightning isolation transformers connected to telecommunications and signalling networks-Selection and application principles