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#65199 - 04/28/06 10:49 AM Short circuit current and older switchgear...
mxslick Offline

Registered: 10/24/04
Posts: 785
Loc: Atomic City, ID USA
In this thread:

my reply to one post brought up a point I feel is worth discussing.

Today's power demands are much greater than they were say, 30 or 50 years ago. This of course required the POCO's to upgrade distribution equipment and feeders to accomodate the growing load. Yet in both industrial and commercial locations, some very old but still servicable switchgear and breaker panels are in use.

The concern I have is that said older gear was designed or spec'd for a much lower short circuit interrupting ability than that which could now available to it.

What are some of the potential consequences of this? I see:

1 - Failure of breakers, fuse assemblies and disonnect switches under a Phase-Gnd/Neut. or Phase-Phase fault, with the failure mode most likely violent;
2 - Mechanical failure of buss and enclosures from the forces generated by #1;
3 - Another consequence is failure to clear a circuit as designed, resulting in additional damge and/or;
4 - Severe risk of damage to property or injury or death to persons; and
5 - Risk of extended outages or downtime while the system is repaired.

NEC makes no mention of this situation, and frankly, it would be impossible to require/enforce the changeout of equipment that is no longer able to withstand greater available short circuit current.

But has there ever been, or should there be, a requirement for a complete "power" survey including ASSC and new load factors when major switchgear is being overhauled/ replaced?

I look forward to hearing from everyone here on this subject....
Stupid should be painful.

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#65200 - 04/28/06 11:05 AM Re: Short circuit current and older switchgear...
Jps1006 Offline

Registered: 01/22/04
Posts: 609
Loc: Northern IL
Is the POCO allowed to upgrade transformers to higher AFC without considering what's down stream?

I thought that if POCO needed to increase capacity they would do it in such a way as to avoid leaving a time bomb in all of the existing services.

#65201 - 04/28/06 11:32 AM Re: Short circuit current and older switchgear...
Bob Offline

Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
Say you have customer that has been in business for 50 years and is served with a 100 kva bank. The utility has a new customer next door and the load requires the utility to increase the bank to 1000 kva. The fault is going up but what is the utility to do. It has to provide the service to the new customer. There is no requirement that I know of that requires the utility to do anything except maybe notify the old customer of the possible problems. The utility probably doesn't even do that.

#65202 - 04/28/06 01:34 PM Re: Short circuit current and older switchgear...
mxslick Offline

Registered: 10/24/04
Posts: 785
Loc: Atomic City, ID USA

That's my main question. The POCO's aren't required to notify existing customers when an upgrade creates this situation. (To my knowledge, anyway.)

The exception would be those facilities served by thier own dedicated transformer I would imagine. But what if, in a given KVA size, the newer TX has a lower impedance? Would the POCO then be responsible for insuring that the downstream switchgear can handle the higher ASSC?

Let's see where this goes...

edited for spelling

[This message has been edited by mxslick (edited 04-28-2006).]
Stupid should be painful.

#65203 - 04/29/06 12:38 AM Re: Short circuit current and older switchgear...

Registered: 12/08/05
Posts: 862
Loc: Titirangi, Akld, New Zealand
Good point you raise there mxslick

In New Zealand we have exactly the same happening all over the place, wheater it is industrial or domestic.

Capacity from supply tranformers is upgraded e.g. 100 kVA to 200 kVA, existing cables very often left in place or new circuits added to TF.
If there are HRC fuses in place I can't see too much of a concern. These usually clear faults up to 80 or 120 kA level.

With MCB's it's different. The main MCB in an industrial switchboard may be able to cope with a higher fault level, the circuit MCB's may not, they are usually rated around 6 kA for clearing a fault and could fail or simply blow up when a fault occurs very nearby the main switchboard, say in the first 2 or 3 meters of the outgoing circuit.

In domestic streets for example, lots of houses on overhead supply still have rewirable fuses at point of entry and on the switchboards, With a higher fault level there could be problems although with OH supplies the cable length will reduce the available faultlevel somewhat.

A 5000 Amps faultlevel is generally quoted for domestic switchboard with a 16 mm² mains.

See what other comments we get on this interesting topic...

Cheers Raymond
The product of rotation, excitation and flux produces electricty.

#65204 - 04/29/06 03:07 AM Re: Short circuit current and older switchgear...
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
It's an interesting point. Here in the U.K. we can use fuses or breakers with a rating less than the prospective short-circuit current so long as there is another overcurrent device "upstream" which is suitably rated to break the fault current. So if you had a supply capable of sourcing 20kA, you could still use 6kA circuit breakers in the panel if there was an HRC fuse first which can break 20kA.

Conveniently, all of our supplies have a PoCo-owned cartridge fuse at the service entrance, just ahead of the meter.

#65205 - 04/29/06 06:57 AM Re: Short circuit current and older switchgear...
briselec Offline

Registered: 12/28/05
Posts: 141
Loc: Brisbane, Australia
Lets say an area's demand has been gradually growing over the years so that it's now putting a strain on the distribution network. Wouldn't you fix the problem by splitting some of it onto a new supply rather than increasing the capability of the existing gear. Seems to me that would be the easier and better solution and it wouldn't create the problem you mentioned. It would also have the benefit of adding some redundancy to the network.


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