Three phase and IBM

Tony Duell ard at p850ug1.demon.co.uk
Sun Mar 20 14:33:30 CDT 2011


> Ground should never carry any significant current; if it does, there is 
> a fault.  The allowable current is varies by jurisdiction and type of 

Some mains filters have capacitors from the current-carrying conductors 
to ground. In this case there may well be an current in the earth wire 
even when there is no fault.

> equipment.  In the US, for Class I (electrically insulated with 
> protective earth ground) information technology equipment, the maximum 
> ground leakage current is 0.75 mA for handheld  devices, and 3.5 mA for 
> other devices.

I made a similar comment a couple of months back, and somebody claimed 
that the RCDs (earyh leakage trips, ELCBs, GFIs, whatever you call them) 
to their computer room were set to trip at 100A. I queried this at the 
time, but was assured such things were not uncommon. I am still a little 
worried.

> 
> Because the ground conductor is not supposed to carry any significant 
> current, it should be at nearly the same potential (voltage) everywhere 
> in the local power network.
> 
>  > Y or D depends on the voltage the equipment needs per "leg"
> 
> Not necessarily.  Voltage requirements should be determined by the 
> specifications for the equipment, which ideally are printed on it somewhere.

Indeed, you have have start (Wye) or delta 3 phase supplies at any 
voltage you like.

-tony



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