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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?
 
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?
 
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
 
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.
 
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?
 
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 .
 
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?
 
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.
 
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.
 
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
 
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.
Ironically I have just received an instruction manuel from the dealer(PRAMAC supplier).Until now I have not had any of my emails replied to as their technical helpline has been "under maintenance".The instructions state that an "insulation monitor" is available if an IT configuration is preferred.Had,nt expected that.Instructions also provide a UK website which I will contact to get their input into my concerns.As always the input from the forum is much appreciated.
 
Ironically I have just received an instruction manuel from the dealer(PRAMAC supplier).Until now I have not had any of my emails replied to as their technical helpline has been "under maintenance".The instructions state that an "insulation monitor" is available if an IT configuration is preferred.Had,nt expected that.Instructions also provide a UK website which I will contact to get their input into my concerns.As always the input from the forum is much appreciated.
At least in the UK (and possibly ROI) the use of IT supplies and insulation monitoring are only for skilled supervised systems where faults are very likely to be detected and corrected in a timely manner. The usual use-cases include ship electrics and operating theatres where you really don't want avoidable trips on a first-fault.

PRAMAC seems to be a decent company and they do offer LPG generators that have some advantages for backup (long life of stored fuel, difficulty of stealing it if in a fixed tank) so I would hope they could offer you a better arrangement.

Modifying one to have the CPC back to the generator and RCD there is possible but probably an issue for warranty, etc.

In our case we needed a 3-phase generator in a relative hurry (for winter, not in 8-10 weeks lead time on new builds seen just now) and we asked Stephill Generators and they were happy to modify an in-stock model that had sockets to put a 100mA delay RCD on the 400V circuit to suit our hard-wired use. However, making it floating-chassis to mitigate open-PEN risks might not be practical if the AVR is referenced in any way to the 12V DC system that is always to the metalwork due to typical internal combustion engine electrics.

A final option you could consider is one of the open-PEN units for EV support, but you would need to consider how it is connected as probably it has to be powered from the generator and so would need to be 'reset' on start-up and that would make any automatic start/change-over tricky.
 
A final option you could consider is one of the open-PEN units for EV support, but you would need to consider how it is connected as probably it has to be powered from the generator and so would need to be 'reset' on start-up and that would make any automatic start/change-over tricky.
That was another option that crossed my mind but as you say it appears by no means simple to arrange.My next step is to contact PRAMAC and ask them ,regarding my concerns about the situation I,m installing their genny under, is "What configuration would you recommend?"
 
Is this generator going to be permanently available for automatic backup? If so and you really are concerned about open-PEN risks could it not be put on some sort of insulated raised plinth so in the unlikely event of an open-PEN fault nobody is likely to be in good contact with true Earth at the same time?
 
Is this generator going to be permanently available for automatic backup?
not automatic.Will be connected to installation via a standard C/O switch
and be switched over manually.
f so and you really are concerned about open-PEN risks could it not be put on some sort of insulated raised plinth
yes it could.I,ve given consideration to that too.And of course an insulated plinth would work very well.Also thought about educating the homeowner about how to check for potential open-PEN faults by using a ncv tester before he touches the genny frame.( plus a good pair of rubber gloves!!!).

But essentially I,m looking to carry out the installation in a manner that is 1) safe
2) harmonizes with accespted best practice
3) does,nt compromise the clients warrenty in anyway
4) Does,nt compromise me in any way.

Whats baffling me a little is that I have yet to read in any genny manufacturers literature that "there could be risks associated with an open-PEN scenario "
It could be me of course been a little over sensitive to the possibility.
 
I'm guessing this is in ROI and I'm aware that there is a "neutralising link" between the supplier's PEN and the local earth rod, etc. However, is it a bit like UK TN-C-S where the N & E are only ever linked in the supply cut-out, or is it more like the USA where there is a N-E link in the first DB?

The reason for asking is the time you need to use a generator is under supply fault conditions and that could well include the open-PEN situation, so if any transfer switch could be isolating the supply PEN and line, leaving the installation on the earth rod and then the generator would also be safe under open-PEN failure of the supply.
 

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