415 V to 240V tranformation

[gamwardo]

I am currently doing a job involving a track mounted m/c with a collector rail/pickup supply consisting of 3 phases (415V) and an earth and want to put a 240V lighting circuit on it. I am thinking of picking up 2 phases and running it through a step down transfomer to get the 240V. Does the 0V leg of the secondary winding have to be earthed or not, what does this acheive?

 

[waross]

On a 415V system you should have 239.6 volts from line to neutral or star point. However if the neutral is not run out, with the phase conductors or cannot be run out with the phase conductors you will have to use a transformer.
One side off the secondary is grounded to insure that in the event of a ground fault sufficient current will flow to trip the breaker, among other reasons.
Yours

 

[RalphChristie]

I suppose you have a 3 phase supply without a neutral. Anyway, waross is absolutely correct.
Regulation in the UK states that for a low voltage system "no impedance shall be inserted in any connection with earth.... other than that required for the operation of switching devices, instruments, control or telemetering equipment."
In other words low-voltage systems must be solidly earthed. Low voltage is defined as exceeding 50V but not exceeding 1000V. I suppose the same would apply to the USA and the rest of the world.