IT configured back-up genny

[LastManOnline]

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?

 

[pc1966]

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.

 

[LastManOnline]

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?"

 

[pc1966]

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?

 

[LastManOnline]

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.

 

[pc1966]

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.

 

[LastManOnline]

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,

Yes.Similar to UK

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.

I,ve been a supporter of this approach for a long time.This idea would (in my experience)..

..solve 100% of open PEN issues in new housing estates

..95% in rural houses

..very large numbers of urban houses.The remainder could have their metallic services rendered "harmless"by installing plastic inserts at the point they enter a home.

I should add that if I was doing this genny install in the UK ,I would feel less concerned.From reading this forum I understand that many of your TNC-S supplies are converted TNC underground supplies.I would consider those significantly safer than overhead supplies.
Secondly, I would imagine that the lead sheath ( or what remains of it after conversion to TNC-S.) would still provide a great earth rod.This should be very effective in dragging down the "touch voltage" occuring during any fault.

 

[davesparks]

I,ve been a supporter of this approach for a long time.This idea would (in my experience)..

..solve 100% of open PEN issues in new housing estates

..95% in rural houses

I too support this approach but it would not solve an open PEN, the open PEN fault would still occur.
What it would do is reduce the danger which arises as a result of the open PEN fault.

 

[LastManOnline]

I too support this approach but it would not solve an open PEN, the open PEN fault would still occur.
What it would do is reduce the danger which arises as a result of the open PEN fault.

Correct.I could have phrased that a little better.

 

[pc1966]

Its not really my area but I think most UK homes in built-up areas (i.e. underground cables) were TN-S and those are now TN-C-S (with the C-S transition at the cut-out for new builds, but potentially elsewhere in the network for older conversions). For rural I guess twin cable overheads would always be TT, but now you see the ABC (Aerial Bundle Cable) where an damage incident is likely to take out both constructors maybe those are going TN-C-S as well.

Something like this could do the generator switching you seem to want:

Here two 4-pole switches are arranged so you have either the mains or the generator, and with the "neutralising link" from the supply not connected under off-grid use. There is then a 2nd link (using the generator-side 4th pole) but never two at the one time, and with the 2nd earth rod at some reasonable distance from the supply earth rod you are always connected to a means of earthing.

If the supply switch's earth pole should fail or be fractionally slower to switch, and if the CU is all RCBO (or has an RCD incomer), you can still meet ADS as a TT arrangement no matter what.

When on-mains the generator chassis is isolated from the PEN circuit, and then earthed when in use.

There are no PEN conductors here bar the supply, all of the others may be linked, but they are not simultaneously current carrying and solely linked to the CPC network. The main gap here is there is no RCD "additional protection" on the generator feed so under UK regs it would have to be hard-wired and not on a socket (if no more than 32A).

Just for discussion as I doubt anyone makes such a configuration!

 

[LastManOnline]

Just for discussion as I doubt anyone makes such a configuration!

Think you have the nuts and bolts of a viable system there.Not allowable at this point in time to break the PEN conductor but it would,nt surprise me at all if that were to be reviewed in the not too distant future

 

[pc1966]

Think you have the nuts and bolts of a viable system there.

Thanks!

Not allowable at this point in time to break the PEN conductor but it would,nt surprise me at all if that were to be reviewed in the not too distant future

Ah, but it is not breaking the PEN conductor (which I agree is a very VERY bad thing to do). It breaks the N & E separately so at no point can a part-open contact allow the CPC to be fed from the live current via the installation.

I know there are regs in the UK about not switching the CPC but that is a fundamental requirement for any EV chargers and related open-PEN fault detection systems, however, I have not really looked in to the specifics of regulations around that.

 

[LastManOnline]

Thanks!

Ah, but it is not breaking the PEN conductor

my apologies.I was a bit hasty in looking over it.Should have known you would have paid great attenion to detail