Re: 50 kVA Pole Pig

To: tesla-at-pupman-dot-com
Subject: Re: 50 kVA Pole Pig
From: Jim Monte <JDM95003-at-UCONNVM.UCONN.EDU> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Fri, 07 May 1999 23:48:48 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme


This is getting a bit off-topic, but since it is a something of a safety
issue, hopefully our fearless moderator will have some patience...

I don't think you really can size a circuit breaker to handle a short
at its terminals since the current that will flow depends on the
impedance and voltage of the source supplying the breaker.  These may
be changed without warning by the local power company.

I have not personally had the pleasure of seeing a circuit breaker
fail to open with over 10,000 Amps flowing, but it does not seem like
the kind of think you want to have happen accidentally.

Jim Monte

>Date: Thu, 06 May 1999 17:29:55 -0600
>To: tesla-at-pupman-dot-com
>Subject: Re: 50 KVA Pole Pig
>Original Poster: fprice3 <fprice3-at-acnet-dot-net>
>
>No problem,  the circuit breaker (CB) feeding the pig is presumably sized to
>handle a dead short at the CB's terminals.  Since the pig is downstream from
>this point, the wiring impedance (between the CB and the pig) will further
>decrease the available current.  The pig itself cannot increase the
>Short-circuit current.
>
>Tesla List wrote:
>
>> Original Poster: Jim Monte <JDM95003-at-UCONNVM.UCONN.EDU>
>>
>> Jason,
>>
>> Those pigs will not be any harder to ballast than their smaller cousins.
>
>> Something you may want to look into with such a large transformer is
>
>> the ability of your circuit breakers or fuses to open if it is
>> accidentally shorted.  This will be determined based on the percent
>> impedance of the transformers, the impedance of the wires leading to
>> the transformer, and the interrupt rating of the breaker/fuse.
>>
>> As I understand these ratings (and if there are any errors, someone
>> please correct me) here is an example assuming a typical percent
>> impedance of 1.5%:
>>
>>   50 kVA / 240V = 208.3 Amps at full load
>>   208.3 Amps / 0.015 = 13,889 Amp drawn by the transformer when the LV
>>     side is connected to an ideal voltage source and the HV side is
>>     shorted.
>>
>> Typical circuit breakers for residential use are rated to interrupt
>> up to 10,000 Amps, and fuses up to 20,000 Amps.  So if the wire
>> supplying the transformer is heavy enough, you may have some problems
>> if you are using circuit breakers.
>>
>> Jim Monte
<snip>