# Resistance of a pole pig

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From:  Gary Johnson [SMTP:gjohnson-at-ksu.edu]
Sent:  Sunday, April 05, 1998 11:31 AM
To:  Tesla List
Subject:  Re: Resistance of a pole pig

At 06:35 PM 4/3/98 -0600, you wrote:
>
>----------
>From:  ESchulz531 [SMTP:ESchulz531-at-aol-dot-com]
>Sent:  Friday, April 03, 1998 5:49 PM
>To:  tesla-at-pupman-dot-com
>Subject:  Resistance of a pole pig
>
>Alfred and all,
>
>     I am getting a 10kva pole pig, I know that if you short them that they
>will draw more then they are rated for, so how would you find there real
>internal resistance(as in wire and reactance) not the current they are rated
>for.
>
>thanks,
>
>Erik
>
The short circuit impedance of a transformer is not the same as the
'characteristic impedance' (the ratio of rated voltage to rated current),
except specially built transformer like neons.  It might be listed on the
name plate as 'percent impedance', with values in the range of 5 to 10
percent.  What that means is that if you bolt a solid short across the
secondary and connect the primary to a strong source (a source capable of
supplying many times the rated current of the pole pig), a current of 1/.1
to 1/.05 times the rated current will flow, until something melts or the
circuit breaker opens.  Electrical engineers for the utility use the percent
impedance of all their transformers to size their fuses.  On average,
lightning strikes distribution lines at a rate of once per mile per year,
often producing a power arc.  The nearest circuit breaker should open the
line, putting the minimum number of people out of power.  If the nearest
breaker doesn't work then another one, further away and with a larger rating
will try to open the line.  Every utility of any size will have an
electrical engineer calculating circuit breaker settings, using percent
impedance of transformers, so this is a number that someone in your area
knows even if it is not on the name plate.  Call the local utility and ask
to speak with an engineer in the Transmission and Distribution section.

For Tesla coil purposes, the percent impedance and short circuit current is
not of a lot of interest, however.  If you short your pole pig, you will
probably blow the fuse on the distribution transformer supplying your house,
and maybe supplying several of your neighbors, requiring a visit by the
utility.  This is not good for public relations!  What you really want to
know is how much power you can get out for what period of time before you
damage your pole pig.  These have a long thermal time constant.  Your local
T and D engineer might be able to give you exact specification for your
transformer, but I would not be surprised by numbers like 150 percent for 1
hour, 200 percent for 5 minutes.  That is, your 10 kVA unit would happily
supply 15 kVA for an hour, or 20 kVA for 5 minutes, with no ill effects.

Gary Johnson (I used to teach this stuff at Kansas State University)

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