Discuss IT configured back-up genny in the Electrical Forum area at ElectriciansForums.net

LastManOnline

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Am installing a bqckup genny in a house with a TNCS supply.
Its will be in use 6 to 8 hrs/year (average yearly DNO outages).

My question revolves around the genny,s supply .I see more advantages for the homeowner in having an IT configured genny than a TNS configured genny.
Am I missing something?
 

davesparks

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What advantages do you see in an IT setup?

I assume this generator will be feeding in to the whole installation and not just a dedicated socket or suchlike?
 
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LastManOnline

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What advantages do you see in an IT setup?
Firstly,I am relatively new to this area and attempting to learn my way.
My reasoning is related to the role a backup genny plays and the circumstances it will operate under.They are very different to those of a standalone one.
The backup genny is designed to operate when the DNO supply is itself experiencing fault conditions.Some fault conditions (Open PEN) can be imported and affect the genny frame if it has a TNS configuration.
This should,nt happen if it has an IT configuration.
I appreciate there are some other trade offs between both supply systems but on balance I fimd myself leaning (strongly) towards an IT configuration
I assume this generator will be feeding in to the whole installation and not just a dedicated socket or suchlike?
Yes
 

pc1966

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Generally small (and cheap & nasty) generators have IT operation and usually no RCD (no real point...) as a means of reducing the risk of a fatal shock if in contact. Typically the assumption is only one load or so, but that is often not true and really not acceptable for a home supply.

The larger generators are normally TN and so have RCD(s) (usually, always if socketed and not some hard-wired only device). But they need an earth rod or similar somewhere as a means of earthing. If you check the manufacturer's instructions and/or ask them they should be able to confirm if it is TN, if they can't answer to your satisfaction then probably not a make to deal with.
 

Lucien Nunes

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Some fault conditions (Open PEN) can be imported and affect the genny frame if it has a TNS configuration.

When the DNO PEN goes open under load, the entire earthing system of the building, and anything connected to it, rises to some unknown potential above true earth, and current can be diverted along CPCs and bonding to anything extraneous that is still at true earth potential. If the generator frame is connected to the MET, it will also rise to that potential. I can see an argument that if the genny is the only equipment outside the building's equipotential zone, it might present the greatest shock risk in the event of the MET being elevated above true earth.

But, technically, the differentiating factor as to whether this occurs is not whether the genny supply is TN or IT, it's whether the frame is connected to the MET. With a TN-S connection of the generator, the neutral of the installation is also connected to the MET, with an IT connection it isn't, but this in itself doesn't change the shock risk from the genny frame to true earth.

The permutations are:
IT with genny frame connected: ADS / RCD protection doesn't work, can get shock from frame to true earth with open PEN
IT with genny frame disconnected: ADS / RCD protection doesn't work, won't get shock from frame with open PEN but might from genny leakage
TN with genny frame connected: protection works, can get shock from frame to true earth with open PEN

What are the relative risks of an installation or appliance fault causing a shock risk from the IT configuration, vs. a DNO fault causing a shock risk at the genny from an open PEN condition? As the electrician, whose risks are you tasked with managing? Are you prepared to take responsibility for disabling the additional protection of the installation?
 
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LastManOnline

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IT with genny frame disconnected: ADS / RCD protection doesn't work, won't get shock from frame with open PEN but might from genny leakage
This is the configuration I,m leaning to.
What are the relative risks of an installation or appliance fault causing a shock risk from the IT configuration, vs. a DNO fault causing a shock risk at the genny from an open PEN condition?
These are the exact 2 questions I,ve been wrestling with for the last 5 weeks or so.

I should mention that the TNCS supply is via an overhead line and the installation is in an exposed coastal location.
I appreciate that the IT configuration carries its own downsides,but I feel the chances of two class 1 appliances (-that are simultaneousely accessible) simultaneousrly developing faults in the short window of yearly outages to be significantly lower than the risk of an open PEN .
 

Lucien Nunes

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two class 1 appliances (-that are simultaneousely accessible) simultaneousrly developing faults in the short window of yearly outages

There are types of fault that could be pre-existing, but not causing any symptoms on DNO power. E.g. genny or its wiring faulted or leaking to true earth e.g. by water ingress. Or washing machine leaking from neutral to earth via its heating element, but not low enough in resistance to trip an RCD because on the TN-C-S the neutral and earth are very close in voltage. In this case only one new fault need arise to create the shock risk.

But I understand your reasoning on the relative probability of the different categories of fault scenario and I agree that it is not quite cut and dried, when you have a large metal object outside the equipotential zone, only somewhat earthed via a rod that won't sink all the system load current without exceeding 50V.

How about the 4th permutation I didn't include: TN-S supply from genny but frame floating. RCDs work, no open PEN shock from genny?
 

davesparks

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I feel the chances of two class 1 appliances (-that are simultaneousely accessible) simultaneousrly developing faults in the short window of yearly outages to be significantly lower than the risk of an open PEN .

It doesn't have to be during the power outage, and it doesn't have to be at an appliance.

A low insulation resistance fault can occur at any time and sit there quite happily on the DNO supply and then cause big problems on the generator.
 

pc1966

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How about the 4th permutation I didn't include: TN-S supply from genny but frame floating. RCDs work, no open PEN shock from genny?
I am guessing the fame would need to be earthed, even if a local rod and not linked to the PME supply, so the generator's RCD is meeting ADS for internal faults.
 
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LastManOnline

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How about the 4th permutation I didn't include: TN-S supply from genny but frame floating. RCDs work, no open PEN shock from genny?
That permutation definately has more appeal.It ticks more boxes than the IT option.I did,nt find any info on this permutation in the forums generator threads ,leaving me a little unsure if it could be used but I will now certainly give it consideration
 

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