55v - 110v centre tap transformer short circuit calculations

It depends on which fault you're considering. A L-PE fault on a 55-0-55V system will be driven by 55V, while a L-N fault (really a L1-L2 fault) will be driven by 110V. It's then a matter of identifying the worst case for each of the situations you need to calculate for - for ADS (if needed) it's probably the time to disconnect on a L-PE fault that's critical, while for energy let-though (protection of conductors) the L1-L2 might be the worst case especially if protection is by MCB rather than fuses. You could probably do worse than calculate everything for each situation and see how the numbers look, to get the feel of things.

3-phase RLV system would be 63V L-PE and 110V L-L so I'm guessing you're not looking at one of those.

   - Andy.

Thanks it turens out that i needed to do it to 110V so the equation would be 110v*cmax / Zs. i reconfimed this with my multi meter. 

I suggest that the 110v case and the resistance of a short lead gets the highest fault current, but 55V and the longest lead on the drum gets you the lowest fault current - and I presume you'd like the breaker to still operate in both cases.

Note that despite BS7671s best efforts to confuse matters, by harmonizing with a load of countries that do not use it, one of the really nice things about 55-0-55 is that during a bolted fault at the end of a long lead, the highest touch voltage is 28V or so, and the usual requirement for a faster than a heartbeat disconnection time for safety of life  can be relaxed, so the projection can be more for preventing the cable catching light. It is a very hard system with which to do yourself an injury.

It is however a pity that many builders seem to trail a 13A socket on an extension lead into a convenient puddle of cement solution, and then plug the transformer in right next to the thing it supplies....

Mike.

When I was a lad in the 70’s and 80’s it was a common belief that you had to run a long 240 volt lead to the transformer then plug the tool directly into the Tx, because otherwise there would be excessive voltage drops is you used long 110 volt extension leads.

There were a considerable number of times I was told I was wrong to use long 110 volt extension leads rather than long 240 volt leads and had the Tx in the wrong place.

Andy "3-phase RLV system would be 63V L-PE and 110V L-L " why was I thinking 63.5V? Ok it`s only half a volt but did that change when we reclassed our 240V to be 230V and 415V to be 400V ?

well 110V  / sqroot( 3) is indeed 63.508 if we want to be fussy, so really we should round up to 64V not down to 63.
I suspect that a volt in 63 or an error of 1.8% need not worry us on a building site. With a real transformer unloaded  it could just as easily be be 66.6 which is a devil of a lot easier to remember.

Mike.

well 110V  / sqroot( 3) is indeed 63.508 if we want to be fussy, so really we should round up to 64V not down to 63.
I suspect that a volt in 63 or an error of 1.8% need not worry us on a building site. With a real transformer unloaded  it could just as easily be be 66.6 which is a devil of a lot easier to remember.

Mike.