Surge arrestors and circuit breakers

[BigBill53]

Hello,
I have a colleague who is causing quite a bit of a stir at the moment. He is insisting that you need to connect a circuit breaker in series with a surge arrestor. He insists it is a new law or standard that he heard about that is coming into effect in my country (Australia).

Since most Australian standards come from changes already made overseas, I was wondering if anybody in other countries have to do this?

I personally think it all sounds a bit odd - a 100kA surge arrestor in series with a 10kA circuit breaker. What's the point if the circuit breaker might blow off the wall with the next lightning strike.

 

[rbulsara]

I presume you are talking about low voltage transient surge suppressors (TVSS). kA ratings for surge suppressors are determined much differently (by manufacturers) than the available SCC currents. Best source is to refer to installation manual of the surge suppressors in question.

Typically, in the USA, TVSS are connected through a breaker or fuse to protect against the shorted TVSS. The kA rating of the breaker needs to be greater than the available SCC current.

Rafiq Bulsara

http://www.srengineersct.com

 

[bacon4life]

Section 285 of the 2011 NEC has 3 class of surge protective devices. Class 1 can go on the supply side of the service disconnect. Class 2 must be on the load side of the first overcurrent device. Class 3 must be protected by a branch circuit overcurrent device, usually at least 10m from the service entrance. Lightning strikes are faster than a breaker will react.

 

[dpc]

The 100 kA rating of the arrester is for currents with durations in mioro-seconds. Arresters can fail shorted and the breaker protects the system from these types of failures.

David Castor

http://www.cvoes.com

 

[sibeen]

First of all I'd just consider it common sense. I don't want to have to shut a switchboard down just so that a TVSS can be replaced.

As to the kA rating, they need to be taken with a grain of salt. Having a quick look at a curve for a 100 kA device for a 60mm MOV, the curve states that the surge arrestor can withstand a 100 kA pulse for 20 micro-seconds and do this only once, after which it needs to be replaced.

The same device can only withstand a 1 kA pulse for 1000 micro-seconds again after which it needs to be replaced.

Always check the data sheets when claims of large kA ratings are made and compare them against the standard short and long pulse curves.

 

[BigBill53]

That's amazing - not the answer I expected at all. I can see what everybody here is getting at though.

I have literally hundreds of drawings on my computer from various projects where the surge arrestors are connected directly to the load side of the main circuit breaker. Sometimes there is an isolator connected in series, but only sometimes.

I guess this is a good example of "Don't do something just because everyone else does it that way".

Thanks

 

[dpc]

Surge arresters on high voltage power system are generally installed wtih no protection due to cost and installation issues. But they often fail in spectacular fashion and you don't want to be in the way when the parts start flying.

David Castor

http://www.cvoes.com

 

[rbulsara]

Bigbill:
Perhaps we should have asked what voltage you are talking about. The original post read like it is talking about LV system, your last post sounds like an HV system.



Rafiq Bulsara

http://www.srengineersct.com

 

[BigBill53]

Sorry for not mentioning the voltage - I have been talking about LV systems (415V).