Telecommunications SPD Definition

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Telecommunications SPD Definition

Telecommunications SPD Definition
THREAD: surge protective device (telecommunications), SPD
This discussion is to formulate a definition for an SPD used on a telecommunication line.
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Discussion
This document reviews the existing SPD (Surge Protective Device) definitions, develops and proposes a definition for an SPD used on a telecommunication line. The sections are:
1. Existing SPD definitions
2. What do and don’t we want?
3. What is a voltage limiter?
4. Putting it all together
5. surge protective device (telecommunications), SPD, proposal
1. Existing SPD Definitions
There are three SPD definitions in the IEEE 100 (see Power & Energy Virtual Community, VC surge (telecommunications) discussion, message 1 at https://www.ieeecommunities.org/power?go=319161 ) and two definitions from the IEC Subcommittee 37A. The most common definition styles used in the IEEE and IEC are:
surge protective device
SPD
A device intended to either limit transient overvoltages or divert surge currents or both. It contains at least one nonlinear component.
[IEEE 100, C62.34]
An assembly of one or more components intended to limit or divert surges. The device contains at least one nonlinear component.
[IEEE 100, C63.48]
a device that is intended to limit transient overvoltages and divert surge currents. It contains at least one nonlinear component
[IEC 61643-1]
a device intended to limit transient overvoltages and divert surge currents. It contains at least one non-linear voltage limiting component.
[IEC 61643-21]
Readers of the VC surge (telecommunications) discussion will immediately see the failings of these definitions for an SPD used on a telecommunications line. The definition usage of the following words creates restrictions or are inappropriate for telecommunications applications:
a) transient: Telecommunications SPDs are expected to protect against a.c. as well as impulse increases of level. Any definition of surge which only states transient or sub-second time periods is misleading for the function provided by telecommunications SPDs.
b) overvoltage: This really means a voltage exceeding the rated voltage. Excessive voltage is the appropriate term.
c) surge: The AC definition of surge has the word “transient” which makes it unacceptable for telecommunications use, see item a).
The dangling sentence about the SPD “containing a non-linear device” looks to be an afterthought, possibly an attempt to exclude filters from being classified as SPDs.
“Contain” means to have within. So a manufacturer could put a 2-cent silicon diode inside a circuit box and claim the box contained a nonlinear device – there isn’t a stipulation that the nonlinear device connects to or acts with the internal circuit.
Further, a fuse and step-function PTC thermistor can also be classified as non-linear devices. Even the capacitors used in a filter are usually non-linear.
Audio buffs who say they can hear distortion introduced by capacitors have had their claims substantiated by the careful measurements reported in the articles “Capacitor Sounds” parts 1, 2, 3 and 4 by Cyril Bateman in Electronics World, July, September, October and November 2002. Dallas Semiconductor Maxim Engineering Journal, Volume 48, provides a circuit, which minimises capacitor voltage-coefficient effects in the article “Do passive components degrade audio quality in your portable device?” A good (?) lawyer could use the proven capacitor distortion (nonlinearity) make a reasonable case to class a filter as an SPD using the above definitions. A wound component with a magnetic core can be classed as nonlinear if the current flow saturates the core material.
Only the IEC 61643-21 phrase “non-linear voltage limiting” provides restrictions on the form of non-linearity.
The stipulation that voltage is limited and currents are diverted is muddled. Surely the prime function of an SPD is to limit voltages that exceed a given threshold voltage. The result of limiting a voltage is often to divert a current through the SPD, but this is a cause and effect situation. Excessive currents can only be diverted if they cause the circuit voltage to exceed the SPD protection voltage threshold level.
Obviously it is not possible to make simple modifications to the existing SPD definitions to give a useful definition for a telecommunications SPD.
2. What do and don’t we want?
a) The definition must state that an SPD is a voltage limiting function.
b) The excessive voltage time period shall not be limited by use of words like “transient”.
c) The definition shall not allow an overcurrent function (e.g. fuse or PTC thermistor) or a filter function alone to qualify as an SPD.
d) To avoid misuse, the SPD function needs to be differentiated from a voltage regulator function.
e) To avoid confusion between components, protector units that plug into a base and complete devices, an SPD shall be specified to have the means of making a direct connection to the circuit conductors.
3. What is a Voltage Limiter?
Signal limiters – noise limiters, speech clippers, peak limiters - have been around a long time (Radio Designer Handbook, F Langford-Smith, 1963, pp 694-699). Signal limiter definitions are often meaningful as an SPD is a more powerful version of a signal limiter. The IEEE 100 and the IEC IEV have quite a selection of definitions:
Limiter
a) a device in which some characteristic of the output is automatically prevented from exceeding a predetermined value.
[IEEE 100]
b) a transducer in which the output amplitude is substantially linear with regard to the input up to a predetermined value and substantially constant thereafter. Note: For waves having both positive and negative values, the predetermined value is usually independent of sign.
[IEEE 100]
c) a device that is used to prevent an analog variable from exceeding specified limits.
[IEEE 100]
d) A non-linear device used for limiting
[IEC IEV]
Limiter circuit
a) a circuit that limits the amplitude of a signal so that interfering noise can be kept to a minimum, or to protect components from excessive stress.
[IEEE 100]
b) a circuit of nonlinear elements that restrict the electrical excursion of a variable in accordance with some specified criteria. “Hard limiting” is a limiting action with negligible variation in output in the range where the output is limited. “Soft limiting” is a limiting action with appreciable variation in output in the range where the output is limited.
[IEEE 100]
Limiting
An action whereby all the instantaneous values of a signal exceeding a predetermined threshold value, or outside two predetermined threshold values, are reduced, all other instantaneous values of the signal being preserved.
Note. - The reduction of instantaneous values may be abrupt or progressive.
[IEC IEV]
These describe pretty well with the action of an SPD except for the word “instantaneous”. On fast rising voltages, due to wiring or technology, an SPD will often have a delay in its limiting action (overshoot) even though the voltage threshold has been exceeded. Putting this factor in gives a consolidated term and definition of:
voltage limiter
non-linear device that automatically restricts the voltage amplitude when it exceeds a predetermined threshold level for more than a given time
(Those who appreciated the duality section in the telecommunication surge discussion will be able to get a two for one here. The dual of the voltage limiter definition being:
current limiter
non-linear device that automatically restricts the current amplitude when it exceeds a predetermined threshold level for more than a given time
Further, a current limiter restricts the current delivered to a node or terminal and a voltage limiter restricts the voltage delivered to a branch or terminal-pair)
4. Putting it all together
Building on the voltage limiter definition and referencing the terminal pair gives:
device that automatically restricts a terminal-pair voltage when it exceeds a predetermined threshold level for more than a given time.
Where does the potentially excessive voltage come from? Answer, caused by an external electrical source. Adding this in gives:
device that automatically restricts a terminal-pair voltage, caused by an external electrical source, when it exceeds a predetermined threshold level for more than a given time.
5. surge protective device (telecommunications), SPD, proposal
Something like the below is better than what we currently have:
surge protective device (telecommunications)
SPD
device that automatically restricts a terminal-pair voltage, caused by an external electrical source, when it exceeds a predetermined threshold level for more than a given time
NOTES: 1. An SPD may have multiple output terminal-pair combinations where the voltage amplitude is restricted.
    2. An SPD is a complete assembly, having the means to connect to the circuit wiring.
This definition is sort of ISO/IEC directive compliant as the definition phrase works in a sentence where the phrase replaces the definition.
To prevent further equipment damage, we fitted a SPD.
To prevent further equipment damage, we fitted a device that automatically restricts a terminal-pair voltage, caused by an external electrical source, when it exceeds a predetermined threshold level for more than a given time.
Comments and wordsmithing please.
Regards
Mick
3 Replies & 6 Comments

    1
Albert Martin Mar 16, 2004
   Mick,
   I’m OK with this, until I get down to Note 2 [following point 5].  Note 2 says “An SPD is a complete assembly, having the means to connect to the circuit wiring”  If I look for the definition of assembly in Websters New Collegiate Dictionary, I find as #1 “a company of persons gathered for deliberation and legislation,  worship, or entertainment”.  So if I substitute this definition in Note 2 I get, “an SPD is a complete company of persons gathered for deliberation and legislation,  worship, or entertainment, having the means to connect to the circuit wiring”; or basically a wired congregation [well known in San Francisco], maybe praying that the surge doesn’t get through?
   Although I think what you have in mind here is something like a base that holds the SPD, I would actually prefer to see some term other than assembly used.  When I think of an assembly, what comes to mind is group of components that are arranged to form a SPT [Surge Protection Thingy].  How about unit instead of assembly?  A Webster definition of unit is, “a single thing that is constituent of a whole” [it’s also defined as a local congregation of Jehova’s Witnesses, but I’ll let that one go].  If we accept this suggestion, then maybe SPT could become SPA [Surge Protection Assembly]
   As a note for future discussion, we probably need to define a coordinating device, and maybe also a current-limiting device, since these may be components of an SPT [or SPA].
   Regards,
   Al Martin

Michael Maytum Mar 16, 2004
   Al,
       As is often the case, we are two nations separated by a common language.
   Using the IEC referenced Concise Oxford Dictionary (COD) “assembly” comes out as:
   the action of assembling component parts.
   a unit consisting of assembled components.
   Obviously the second phase meaning was intended
   If you read on in the entry for “assembly” in my Merriam-Webster's Collegiate Dictionary (Deluxe Audio Edition) item 5 states:
   a)   the fitting together of manufactured parts into a complete machine, structure, or unit of a machine
   b)   a collection of parts so assembled
   These definitions are not too far away from the COD definitions.
   The term “unit” is likely to cause confusion in the US as Telcordia has several GRs on TLPUs (Telecommunication Line Protection Unit). You cannot usually fully test these “units” without using a holder or base to provide the link between the TLPU pins and the test circuit wiring. In other words, a TLPU is not an SPD. The whole thing, TLPU and its base (giving the wiring connection) form an SPD. At high current and temperature levels, the base can be the performance-limiting item. The SPD should be tested as it is installed in the field.
   I hope I’ve made this clear enough; an SPD is the device you wire up to and test. How to make note 2 clear or clearer? Here is another attempt, but as I requested, we really need a wordsmith or two to help us out.

   2. An SPD incorporates the appropriate terminals or connections for connecting to the circuit wiring.
    
   Regards
   Mick

William J Curry Mar 17, 2004
   Mick,
   RE: Note 2. in your Mar. 16 response to AL
    2. A SPD provides a means of connecting to the circuit conductors.
   The term "terminals" (how's that?) might for some exclude certain means of making electrical connections.
   We may want a SPD to make contact with a conducting structure other than "wiring".
   Bill
   p.s. I'll leave it to others to decide upon "A SPD" or "An SPD"

Michael Maytum Mar 18, 2004
   Bill,
       Terminal is a generic connector, as defined by the IEC and IEEE, so could be used.
    
   terminal
   conductive part of a device, electric circuit or electric network, provided for connecting that device, electric circuit or electric network to one or more external conductors
   IEC IEV
    
   terminal
   A conducting element of an equipment or a circuit intended for connection to an external conductor.
   IEEE 100
    
   Using conductor is good, being generic it can be anything from a wire to a large bus bar.
    
   conductor
   element intended to carry electric current
   IEC IEV
    
   conductor
   A substance or body that allows a current of electricity to pass continuously along it.
   A material, usually in the form of a wire, cable, or bus bar, suitable for carrying an electric current.
   IEEE 100
    
   Regards
   Mick

    2
MICHAEL PARENTE Mar 17, 2004

   Mick  --
    
   Here are some suggestions for the definition of telecom SPD.
    
   a.  Can we delete “automatically”?  I don’t think it is needed, and is slightly mechanical in tone.
    
   b.  We should be able to delete ‘for more than a given time”.  If a SPD, say a gas tube, overshoots on fast-rising waveforms, then its specification provides a predetermined threshold level for the overshoot.  Else, there is the implication that the overshoot can be arbitrarily high until the given time is reached.  Said differently, there may well be more than one predetermined threshold depending on the test waveform, so the “given time” portion of the definition is not needed.
    
   c. Make use of our recently adopted term “telecom surge”.  Substitute it for  “external electrical source”  This change also provides the tie-in to “S” of SPD.
    
   d. The whole question of current limiters should be more directly addressed for telecom SPDs.  Else, we will continually be open to arguments as to including current limiters in our standards.  Keep in mind that there are probably more than 100 million telecom SPDs in service that contain current limiters.
   The statement is not always true that “excessive currents can only be diverted if they cause the circuit voltage to exceed the SPD protection voltage threshold.”  Heat coils operate on voltages that are driven by sources too low to operate the voltage limiter (often 120Vac), yet they divert currents.  While they limit surge currents in the protected load, their shorting action may increase current in the telecom line.
   As shown in the Basic Configurations of an SPD in C62.31 (Fig 1), we have long agreed that a SPD must have a voltage limiter function, but it may also have a current-limiter function.  This function should be acknowledged in the definition of telecom SPD.
    
   e.  These suggestions result in the following:
    
   Surge protection Device (telecommunications):
   device that restricts a terminal-pair voltage, caused by a surge on a telecommunication line, when it exceeds a predetermined threshold level.  It may also restrict terminal-pair current to a predetermined level.
   Michael Parente

Michael Maytum Mar 28, 2004
   Mike,
      My comments on your posting #5 are:
    
   a.  Yes, I think we can delete “automatically”.
   automatically
   to
   automatically
    
   b. The expression “for more than a given time” allows for certain deficiencies in the practical product. However, this becomes difficult to interpret for dv/dt initiated SPDs (These can exist, least I filed a patent on one, many years ago.)
   To bring in the idea of a waveform dependent threshold would fix this as you suggest. I can’t warm to “waveform-dependent predetermined threshold”, there sounds to be too many options here. Maybe it would help to leave things fuzzy, but leave it open for people to suggest improvements.
    
   c. When I first worked on the definition, I included telecommunication surge. Then I became concerned that telecommunication surge definition might not Curry favour (J) at the 3.6.7 meeting. This is why I made the definition independent of the acceptance of the telecommunication surge.
   However, these things can be swapped about and I am happy to use surge in my original expression.
    
   You suggested replacing:
   caused by an external electrical source
   by
   caused by a surge on a telecommunication line
    
   That’s fine by me.
    
   d. I appreciate your comment about current limiters. At the time of writing I was highly polarised not to have an SPD definition that might be met by just a current limiter. As a device, the prime function of an SPD is voltage limiting. Other functions, like current limiting and filtering are secondary functions and should appear as a note, if needed.
   A current limiter is only guaranteed to operate at an output or input terminal, as the current return path may not be through another SPD terminal. (Duality- voltage limiting across a terminal-pair or branch, current limiting through a terminal or node)
   How about a note?
   NOTE: Secondary functions may be incorporated, such as a current limiting to restrict a terminal current.
    
   Putting this all together and including Bill Curry’s contribution gives:
   surge protective device (telecommunications)
   SPD
   device that restricts a terminal-pair voltage, caused by a surge on a telecommunication line, when it exceeds a predetermined threshold level
    
   NOTES:  1. An SPD may have multiple output terminal-pair combinations where the voltage amplitude is restricted.
   2. An SPD is a complete assembly, having terminals to connect to the circuit conductors.
   3. Secondary functions may be incorporated, such as a current limiting to restrict a terminal current.
    
   Regards
   Mick

    3
   Francois Martzloff Mar 25, 2004

          
   First, thanks go to Mick for his note calling attention to the problem of tiny fonts on some VC chat screens.
   The following text appears in your saga of telecommunications SPD discussions:
   "The stipulation that voltage is limited and currents are diverted is muddled. Surely the prime function of an SPD is to limit voltages that exceed a given threshold voltage. The result of limiting a voltage is often to divert a current through the SPD, but this is a cause and effect situation. Excessive currents can only be diverted if they cause the circuit voltage to exceed the SPD protection voltage threshold level."
   If yours truly (an old-time AC power circuits character) may be permitted to chime in your telecom discussions, I would like to object to the philosophy implied in the comment that I cite above.
   Indeed, this is a cause-and-effect situation, but it can be seen the other way around: The transient "suppression" function of a varistor is the result of the continuous, monotonic response of the varistor to a current being delivered by a surge to a voltage divider consisting of the source impedance of the surge and the decreasing impedance of the varistor.  There is no "threshold" effect involved.  As the impedance of the low side of the voltage divider (the varistor) decreases progressively while the the high side of the divider (the linear source impedance between the surge source and the varistor) remains unchanged, the ratio of the divider changes and the low side of the varistor maintains ("clamps") a relatively low voltage across itself -- and the parallel-connected load to be protected.
   I see in the questionable perspective of the cited paragraph the same misconception of varistor operation that has led -- in some recent discussions of varistor failure modes -- to making reference to a certain level of TOV voltage at which "the MOV begins conduction" (!#*^&$@!!)  Those of you who have witnessed these discussions by now know my response: an instant offer of a cake of mouth-washing soap to the person who utters the words "the MOV begins conduction." I do not know how to send a physical cake of soap via the VC, but consider my message (thank you for your patience in reading my harangue) as a virtual cake of soap !
   In conclusion, perhaps making it an epigram:
   For voltage-switching SPDs:  "Threshold" -- YES !
   For voltage-limiting SPDs:  "Threshold" -- NO WAY !
   With my best wishes for you telecom gals and/or guys for progress in your efforts.
   François Martzloff
   March 19, 2004

   Michael Maytum Mar 26, 2004
   François,
         Thanks for your comments and virtual cake of soap – I shall use it as appropriate.
   An MOV has a continuous v-i characteristic from zero; there isn’t any dispute on that. It’s hard to make definitions perfect, but I think most people in telecommunications engineering would understand the context of “threshold”.
    
   In the twelfth century, “threshold” was the plank, stone, or piece of timber that lies under a door, marking a level or point at which something would start or cease to happen or come into effect. Here, threshold means a demarcation point or boundary.
    
   The other use of “threshold” is as point at which an effect begins to be produced or detected or considered to happen. This can be subjective as in “pain threshold”. The guy feels something before the “pain threshold” is reached, but after the “pain threshold” then things are uncomfortable. Here something is happening below the pain threshold, but it is tolerable.
   Likewise on the varistor, although there is attenuation below the “threshold voltage” it is tolerable along with the resultant dissipation – otherwise people would not use varistors.
   Technically, “threshold” can be used for straight-line engineering or establishing a level. Here is an example of each use from IEC 60050:Part 521 (IEV).
    
   a) threshold voltage (diode or thyristor)
   value of the voltage at the intersection of the straight line approximation of the forward (on-state) current-voltage characteristic and the voltage axis
   (straight-line engineering)
    
   The forward voltage characteristic of a junction diode is an exponential relationship with forward current. On a linear current scale, very little current flows until about 0.4 V. Increasing the voltage above that value rapidly increases the current flow. Within a particular current range the forward diode v-i characteristic can be approximated as a (threshold) voltage in series with a resistor.
    
   b) threshold voltage (enhancement type field-effect transistor)
   gate-source voltage at which the magnitude of the drain current reaches a specified low value
   (establishing a level)
    
   Here, the voltage is adjusted to achieve a specified current.
    
   I hope this explains the how “threshold” is used in the proposed definition.
    
   Regards
   Mick
   PS Like you, I too have some “hot spots”
   One particular one is when people talk about
   “voltage-switching SPDs” and “voltage-limiting SPDs”
    
   Surely ALL SPDs, both switching and clamping/clipping types, limit the voltage? A switching-type SPD and a clamping/clipping-type SPD both limit the voltage, but do it in different ways. An orchestra and jazz band both make music; they too do it in different ways. It’s sad when etymological inexactitudes of the past in AC protection continue to get perpetuated, but I consider it unacceptable when people try to force them into telecommunications usage.

MICHAEL PARENTE Mar 27, 2004
    
   Can the issue be resolved by simply removing "threshold" from my proposed definition of March 17?
    
   Surge protection Device (telecommunications):
   device that restricts a terminal-pair voltage, caused by a surge on a telecommunication line, when it exceeds a predetermined level.  It may also restrict terminal-pair current to a predetermined level.