Neutral earth switch sizing for multiple generators
Neutral earth switch sizing for multiple generators
(OP)
Guys,
Looking through the archived Q+A's, I am unable to find a definitive answer to "How to correctly size a Neutral earth switch".
We are working under UK's G59 regulations, just to be clear.
I fully understand the reasons behind NE (or star point) switching, but I am unable to find reasoning behind switch ratings.
It's clear that under normal operation that there is little or no current flow through the switch, so long term current carrying capacity is not so important.
We assumed that we should have the NE switch the same rating as the main generator switch (also with the same fault rating), but is this the actual case?
We have multiple generators that run island parallel, so it's clear to me that only one star point should be grounded at this point and no generator star point grounded when parallel to mains for any duration.
The question is, do we have to take into account the second alternator into the switch rating.
I believe that the answer is "Yes", so do I size based on maximum PSC of both alternators???
Any advice gratefully received.....
Looking through the archived Q+A's, I am unable to find a definitive answer to "How to correctly size a Neutral earth switch".
We are working under UK's G59 regulations, just to be clear.
I fully understand the reasons behind NE (or star point) switching, but I am unable to find reasoning behind switch ratings.
It's clear that under normal operation that there is little or no current flow through the switch, so long term current carrying capacity is not so important.
We assumed that we should have the NE switch the same rating as the main generator switch (also with the same fault rating), but is this the actual case?
We have multiple generators that run island parallel, so it's clear to me that only one star point should be grounded at this point and no generator star point grounded when parallel to mains for any duration.
The question is, do we have to take into account the second alternator into the switch rating.
I believe that the answer is "Yes", so do I size based on maximum PSC of both alternators???
Any advice gratefully received.....
"Power is nothing without control"






RE: Neutral earth switch sizing for multiple generators
RE: Neutral earth switch sizing for multiple generators
"Power is nothing without control"
RE: Neutral earth switch sizing for multiple generators
These values are determined by the switchgear designer and are part of the system protection and selectivity design.
Only the first generator closes its neutral switch when, energised, closes the main breaker to dead bus.
RE: Neutral earth switch sizing for multiple generators
Once you have gone through scenarios like this then the answer, and justification as to size falls out.
RE: Neutral earth switch sizing for multiple generators
It all depends on the rest of the system, but it is typical in the systems I have seen, to have a single neutral resistor, rated to keep the phase to earth fault current to the same value as line current. This seems to be a compromise between giving sufficient current for discrimination of protection etc and minimising any fault damage.
I thing they use HV vacuum contactors, because these are the most appropriate device, being robust and capable of remote operation. I guesss that this will be rated well in excess of what you need, since the fault current is limited by the resistor. So there is not a complex calculation to size the "switch".
It is practice to have all switches closed when the generator is at standstill and up to synchronisation (to make sure that the generator earth fault protection sees an earth fault before you connect it to the network).
RE: Neutral earth switch sizing for multiple generators
I would state Kirkoff's law as the authority to size the neutral switch the same as the generator main switch.
Bill
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"Why not the best?"
Jimmy Carter
RE: Neutral earth switch sizing for multiple generators
A sketch of the proposed switching and earthing arrangement would go a long way to clarifying things.
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If we learn from our mistakes I'm getting a great education!
RE: Neutral earth switch sizing for multiple generators
RE: Neutral earth switch sizing for multiple generators
"Power is nothing without control"