LV Tie CB Short Circuit level 1

[jps1]

On our Transformer LV side I have SC currents of up to 50kA. The transformers are typically 1MVA 3.3kv to .42kv. They were manufactured in the 1950 to early 1960 and have a %Z of 1.92.
The secondary tie CB are normally operated open. When close, the SC level goes up to approx. 100kA.

My question is this, base on the NEC or IEEE standard, is there a requirement to have the LV switchgear braced for 100kA? The tie CB is rarely closed and it done only when one transformer is taken out for maintenence.


Regards

 

[dpc]

Yes, that is the basic requirement. The NEC offers no exceptions based on short duration of parallel operation. The NEC says that all equipment must be rated for the maximum possible short circuit current. So even a temporary tie would violate the NEC requirements. The bracing is actually not as critical to safety as the interrupting ratings of the circuit breakers involved.

You could ask the AHJ for an variance. Generally, some type of interlocking or alarms are provided to limit the length of time the two transformers can be paralleled.



David Castor

http://www.cvoes.com

 

[Zogzog]

Well you are both right, the AHL makes the call but generally what I have seen allowed is only closed transition systems when there is some sort of relaying involved that will not allow one to "forget" to open either a main or the tie after a few seconds, some call them "Electronic key interlocks".

But for older gear most M-T-M subs have physical kirk keys. Depending on the breaker type there may be a higher interupting version available if you insist on a closed transition sequence.

 

[dpc]

Erminio,

The NEC rule is sometimes violated in practice in the US. Also, the electric utilities in the US are exempt from the NEC and have a lot more flexibility in this regard. The NEC is intended to protect installations where there is no trained electrician or engineer on site. We generally specify a keyed interlock for non-utility applications to prevent parallel operation. Or at least make it more difficult




David Castor

http://www.cvoes.com

 

[jps1]

Thanks for the advice. I was also thinking that I needed to cover for the highest possible SC current.

 

[Bramb]

David, Zogzog
thanks for your explanations.
I asked to delete my post as jps1 was asking for reference to standard and I wasn't able to give precise ones.

I want to check about it in IEC standards, but unfortunately I haven't get it yet.

Thanks
Ciao
Erminio

The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.