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Posted By Topic: Generator RCD

weedieone
Apr 11 2019 11:59

We have a client that has a generator inlet on the side of their house to feed the switchboard during power outages. The inlet is connected via a change over switch and all is wired correctly as per figure 7.4 in AS/NZS3000:2007.

The problem is the clients generator has an RCD and the MED link in the house causes this RCD to trip (as it should in this situation).
Can the RCD in the generator be removed?

This is the first time I have seen this as most people have had generators without RCD's.

Cheers for your help
   

AlecK
Apr 11 2019 12:22

For this sort of application, you want an isolated output, not an RCD-protected output. Just removing the RCD woun't be enough, you need to not have the winding connected to earth at all; else load current will flow in the PEC of the supply, because there will be parallel paths.

If connected at one end of the winding, by a special link designed to be removable, then lifting this link may make the genset OK for use to supply an installation. But will be "less safe" for use for other purposes, and will probably void the guarantee.

But if the middle of the winding is earthed (centre-tapped), so the genset provides a 2-phase 110 V supply with phases at 180 degrees; then get rid of it and buy another genset that's actually suitable.
   

DougP
Apr 11 2019 13:00

Inverter generators are usually isolated output.
   

pluto
Apr 11 2019 14:02

If an inverter generating set is being considered it is moet unlikely to be a centre tapped alternator. Take the RCD away and replace with a MCB. The electronic inverter will usually have an electronic based overcurrent protection which will be quicker than an external MCB. But check the manufacturers handbook to verify this before changing the protective device.

If an RCD has been fitted a frame to end end of the output winding connection would have been made externally to make the RCD test button and for normal operation for the RCD to operate under earth fault conditions.

My (and your reading) of reading of the fundamental outcomes of AS/NZS 3000 (clauses 1.5.XX) shows that an isolated output is a class of protection permitted without further protection and provided that only ONE class I appliance is being supplied, it is a reconized system of supply.

An isolated output is much safer than using an RCD on it's own which is only classified as additionasl protection and not suitable to be use as the only protective device.
   

pluto
Apr 11 2019 14:08

Also note the warning in AS/NZS 3000 clause 7.3.8.1.1 (a) about the removal of any N to E strap inside the generating set.
   

AlecK
Apr 11 2019 15:41

"provided that only ONE class I appliance is being supplied"

Exactly.

Also reliant on actually maintaining the isolation under field conditions.
   

DougP
Apr 11 2019 19:12

I had a question from another sparky..

Who asked about the case of using an isolated generator to supply a MEN installation, what happens if someone plugs in something else to the second socket... like an extension cord to the neighbours fridge for example..

"good point" I said...
   

pluto
Apr 12 2019 07:21

DougP Apr 11 2019 19:12

Your comment

I had a question from another sparky..

Who asked about the case of using an isolated generator to supply a MEN installation, what happens if someone plugs in something else to the second socket... like an extension cord to the neighbours fridge for example..

"good point" I said...

My comment
If you draw out what the as/nzs 3000 fig 7.4 connection details and add another load (like an extension cord to the neighbours fridge from above) if connected to the second socket outlet on the generating set.

The generating set output neutral and frame are seperately earthed from the electrical installation neutral and earth busbars it is supplying.

The second load overcurrent protection will be provided by the generating set overcurrent protection device.

This not electricly unsafe with this mode of connection.

I always use a bit of paper and a pen and draw it out to what the proposed connection(s) would give to check to see what would happen if carried out in practice.