Generator earth
Generator earth
(OP)
What is the reasoning behind the 10ohm maximum impedance specification (low voltage generators in UK )for a gen set connection of its neutral point to earth? I trust I understand the reason for the connection but I would be interested to know just why the figure of 10 ohms was insisted upon.
Regards,
Lyledunn






RE: Generator earth
As per IEC the fault current seen by the gen 51G relay must be atleast 3 times the max e/f current seen by the largest outgoing feeder setting or the fuse size. This is to ensure that the gen relay has adequate margin for it to operate.
You do not say how big the gen is but I suspect that the 10ohms will limit the e/f current to atleast the gen FLC if not a bit higher.
RE: Generator earth
Regards,
Lyledunn
RE: Generator earth
RE: Generator earth
This low resistance is sort of a compromise between no resistance at all and high resistances.
RE: Generator earth
I see where you are coming from, my apologies, I didnt explain fully. Take a typical say 20Kva 400/230v gen set. It has 3xphase, 1xneutral and a separate dedicate earth conductor, (no earth fault relay). The current generated due to any fault to earth will be carried by that dedicated earth conductor. In this case the required connection of the neutral point of the set to earth then has little to do with limiting fault current for the purposes of discrimination as you desribe (I take it that you were assuming no separate dedicated earth conductor when you made your response).The purpose of the connection of the star point to earth is to establish a reference and ensure that the neutral conductor remains at or about earth potential. I was just wondering why 10 ohms was set as an upper limit.
Regards,
Lyledunn
RE: Generator earth
The requirement for an upper limit of 10ohms is to ensure that the gen can diliver a maximum amount of earth fault current in the event of a e/f.
RE: Generator earth