Using fuses as TVSS disconnect

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Joined: 6 Jan 2011 - 19:02
Using fuses as TVSS disconnect

Using fuses as TVSS disconnect
Dear Colleagues:
My company installs TVSS products in our transfer switches from several different suppliers. One supplier in particular does not offer an integral disconnect switch with its 200KA (SCCR) 277/480V 240KA/PH TVSS unit. We require a disconnect for servicing the TVSS while the load is connected to the utility. It has been suggested to me to use a pop-out 3-pole fuse holder with 30A KTK fuses as the disconnect device, thus consuming as little panel space as possible inside the ATS enclosure. Can I please get some feedback on this? Does the use of a fuse in the serial conductor path simulate adding length to the conductor and thus degrading the TVSS Suppression Voltage rating?
4 Replies & 8 Comments

J Koepfinger Oct 17, 2008

The term TVSS has now been eliminate in UL 1449. It is called a SPD (Surge Protective Device. Any recommendation regarding the fuse to protected the SPD should come from the manufacturer. He should be able to give you a recommendation. You may wish to ask him what is the value of the measured protective level at 200 kA for a standard lightning impulse current of 8/20 microsec and 10/350 microsec impulse current waveshape. You may find that the measured protective level at 200 kA is going to be significant compared to any voltage drop across the fuse. It is suggested that you might find it use fully to get a copy of IEEE 62.41.1. This standard provides some very useful information on the maximum surge current that can be expected in a low-voltage distribution system. All SPDs must either have internal protection or external protection recommend by the manufacturer and have been tested with that protection in place by a certified laboratory.


Daniel R Casey Oct 21, 2008

Since my original post on this subject, I have been informed that the significance of lead length for Type 2 SPDs is not a factor of wire resistance but rather of wire inductance where the voltage drop of the conductor lead portion during a surge event is determined by V=LxdI/dt.
Can someone point me to a relevant real-world calculation example or text that illustrates this point? I do not have access to IEEE C62.41.1, so I don't know if I can find the answer to my question there. I would like a clear explanation with the use of formulas to show how the inductance is developed in a piece of straight #6 stranded wire given a typical surge from a lightening strike, a bolted fault, or electrical switching noise. The purpose here is to better understand the need for SPDs, and how to best select from available suppliers. What is the typical, voltage and current during a surge from each of these scenarios?
The SPD application for this discussion is type 2, connected to the load side of a service entrance rated overcurrent device using 18" of #6AWG MTW for the phase, grounded, and grounding conductors. The SPD is rated 200KAIC overall SCCR and is able to shunt 120KAIC L-N and L-G (240KA/Phase).

Doug McCall Nov 3, 2008
You are correct about the inductance of the cables. However, C62.41-41.1, .2, and -.45 will not assist you in a calculation, but may help in understanding some aspects. UL 1449 3rd Ed. may help somewhat also. Re a calculation, any competent electrical engineer should be able to help with that.

Daniel R Casey Feb 6, 2009
Gentlemen: I am still waiting for anyone to give me a specific and detailed reply to my 10/21/08 12:13PM entry on this discussion post.
 Let me restate the question again:
How is inductance produced in a straight piece of wire during a surge event for a Type 2 SPD (2008 NEC 285.24) , i.e., how is the voltage drop developed on the arrester leads? I would appreciate a link to an article that explains this.
Secondly, what is the expected surge voltage and current from the various sources to be protected from (lightning external to a building and transients from switching motor loads and lighting inside a building - adjacent branch circuits as well as down stream?
Thank you for any help.

XIE WATER Apr 17, 2009
  I have UL1449 3rd standard. If you require pls contact me.

Daniel R Casey Apr 21, 2009
  Response is to "SETFUSE"... please provide UL1449 3rd standard. I do not have your contact info, so here is mine: or (440) 232-0200 x-225.

Michael Maytum Apr 26, 2009
The paper version of UL 1449 Edition 3 is nearly 1000 $. UL 1449 Edition 3 is a purchased product; it is not free like the documents from organisations like the ITU-T or TIA-JEDEC.
Other than UL to its subscribers, any other person giving out this document violates UL 1449 Edition 3 copyright:
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form by any means, electronic, mechanical photocopying, recording, or otherwise without prior permission of UL.
The IEEE, PES and SPDC don’t condone copyright violation. Full draft or published standards created by the SPDC may only be posted in restricted access areas of this Forum. Any materials that are in copyright violation will be removed from this Forum.
Edition 3 change summaries can be found by Web search, examples are: 1449ThirdEd paper.pdf

Daniel R Casey Apr 27, 2009
Thank you sir for making this clear. I will not violate any copyright laws and stand corrected as to my request for this UL document.  I will comply with IEEE standards of conduct at all times.
However, no one has provided an answer to my question regarding how the SPD conductor leads develop a voltage drop inductively when they are a straight piece of wire. I am interested in this concept for surges ocurring from lightning strikes (external to circuit and upstream of SPD), bolted faults (internal to circuit and downstream of SPD), and transients from switching loads (both upstream and downtream causes, i.e., motors and lighting).

Ron Hotchkiss Apr 27, 2009
The following link should provide all the information and references required for one to understand why lead length causes degradation in performance of parallel (shunt) connected SPDs.
This is a paper by Francois Martzloff and Kermit Phipps which provides both a theoretical explanation as well as practical experiments to make the point. It is an excellent explanation of the topic.

Daniel R Casey Oct 24, 2008

Please can someone tell me how to obtain the IEEE standards for SPDs? Thanks.

Doug McCall Nov 3, 2008
IHS Global

Dalibor Kladar Oct 26, 2008

The design of SPD disconnect (fuse or CB) is responsibility of SDP designers (SPD manufacturers not SPD users). The first design goal is to coordinate damaging Time Current Characteristic (TCC) of SPD with TCC of SPD disconnect. For more details see United States Patent US6636409.
SPD suppressed voltage rating is influenced more by inductance of connecting wires than by the fuse itself.