High Ze on PME question

Gavin John Hyde · Dec 22, 2018

Come across this on instagram this morning.
Any thoughts? Dont have much experience of generators myself,
In terms of this question my thoughts were additonal or secondary supply source with a rod skewing readings? Not sure though. Would somebody with experience of generators explain?

[ElectriciansForums.net] High Ze on PME question

Yozza19 · Dec 22, 2018

Change over contactter ?

Strima · Dec 22, 2018

Says PME then says generator?

What size generator and type?

Yozza19 · Dec 22, 2018

You would normally expect a lot less if it is directly generator fed

It could be something to like a generator through a transformer/ inverter

Or floating n in generator

Yozza19 · Dec 22, 2018

I think the op is saying mains installation is pme

That is the problem with back up generators people fit them and do not cheak readings and results ensuring breakers / switchgear would be suitable when on back up

Gavin John Hyde · Dec 22, 2018

Dont know more than in that instagram post shown in image. It confused me, hence why i put it on here for peopme to mull over.
I read it as pme supply but the generator on site some how connected to same system?

Yozza19 · Dec 22, 2018

The Cpc of the generator is usually permanently connected and usually the generator go through a 4 pole contractor with lock out switch

Deleted member 105166 · Dec 22, 2018

I imagine the standby generator has its own local electrode and the CPC of the installation is connected both to the DNO's PME and to the generator's electrode (plus bonding to any other conductive parts in between such as the changeover switch cabinet, generator casing and armouring of the SWA between the genset and the c/o switch.)

Is the 1.96Ω reading a Ze of just the DNO supply with all parallel paths removed, is it the DNO supply plus the standby set with the complete earthing arrangement, minus other parallel paths?

Richard Burns · Dec 22, 2018

You can't have the generator earthing arrangement connected to the DNO supply because an internal fault when running on the generator may direct voltage back to the DNO supply on which someone may be working.
Therefore I would expect a changeover switch has been operated to take power from the generator and the Earth resistance for the generator earthing is 1.96Ω.
The PME supply is cut off by the changeover switch.

Deleted member 105166 · Dec 22, 2018

Richard Burns said:
You can't have the generator earthing arrangement connected to the DNO supply because an internal fault when running on the generator may direct voltage back to the DNO supply on which someone may be working.

Understood, @Richard Burns So is my understanding correct as below, on the basis the changeover switch only switches the two or four live conductors (depending on whether single or three phase) and not the CPC:

Generator enclosure & SWA = TT
The installation always utilises the DNO's PME at all times, irrespective of position of the c/o switch
The changeover switch cabinet should be bonded to the PME, as this earth will be of lesser resistance
The SWA armouring from the generator to the c/o switch should only be terminated at the supply end and specifically NOT terminated at the c/o cabinet inlet, to ensure separation of the local and DNO earthing paths

Deleted member 26818 · Dec 22, 2018

The earth fault loop path for whichever supply is being used, must be complete.
When the DNO supply is in use, the generator supply becomes an extraneous conductive-part, and as such requires bonding to the DNO supply earth.
The only acceptable method to divorce the two supply earths is to use a changeover switch which will disconnect the DNO earth at the same time as the live conductors.

Deleted member 105166 · Dec 22, 2018

spinlondon said:
The earth fault loop path for whichever supply is being used, must be complete.
When the DNO supply is in use, the generator supply becomes an extraneous conductive-part, and as such requires bonding to the DNO supply earth.

That was my original understanding, however I do understand Richard's scenario, which is essentially the reverse of the often discussed issue of exporting a PME earth outside its equipotential zone and the difference in potential between the exported earth and mother earth, under a fault condition on the DNO side. In this case, the issue is a fault on the client side at the generator, with the DNO operative subject to the rise in potential at the DNO end of the PME.

spinlondon said:
The only acceptable method to divorce the two supply earths is to use a changeover switch which will disconnect the DNO earth at the same time as the live conductors.

Most changeover switches, certainly all those that I've seen, only switch live conductors. I understand you could use switch the supply earths, however this doesn't overcome the extraneous aspect.

Strima · Dec 22, 2018

Most generator set ups I’ve been involved with that weren’t automatic had the earth permanently connected with a link on the earth bar that had to be removed when switching the generator over.

Not sure about AMF as I’ve never really worked on them.

Richard Burns · Dec 23, 2018

TonyMitchell said:
Understood, @Richard Burns So is my understanding correct as below, on the basis the changeover switch only switches the two or four live conductors (depending on whether single or three phase) and not the CPC:

Generator enclosure & SWA = TT

The installation always utilises the DNO's PME at all times, irrespective of position of the c/o switch

The changeover switch cabinet should be bonded to the PME, as this earth will be of lesser resistance

The SWA armouring from the generator to the c/o switch should only be terminated at the supply end and specifically NOT terminated at the c/o cabinet inlet, to ensure separation of the local and DNO earthing paths

I am thinking that you could for example have something like the below diagram.
When the TNCS supply stops the changeover switch is operated, the earth link removed and the generator started.
When the TNCS supply is restored the generator is stopped, the earth link replaced and the changeover switch operated.

When running from the DNO supply the generator earth is bonded as an extraneous conductive part to the supply earth by the generator earthing conductor.
When running from the generator the link is removed so the Line, Neutral and Earth from the DNO supply are isolated from the installation.

As I was drawing this I have realised that the alternative solution to the question in the OP is that the earth link has been removed from the installation even though the DNO supply is still connected leaving the installation as a TT system. Assuming the protection for the installation is correctly set up to be suitable for either supply this should not cause danger to the installation but would be a less effective means of protection than to use the DNO supply earthing.

Deleted member 26818 · Dec 23, 2018

Richard Burns said:
I am thinking that you could for example have something like the below diagram.
When the TNCS supply stops the changeover switch is operated, the earth link removed and the generator started.
When the TNCS supply is restored the generator is stopped, the earth link replaced and the changeover switch operated.

When running from the DNO supply the generator earth is bonded as an extraneous conductive part to the supply earth by the generator earthing conductor.
When running from the generator the link is removed so the Line, Neutral and Earth from the DNO supply are isolated from the installation.
View attachment 46636
As I was drawing this I have realised that the alternative solution to the question in the OP is that the earth link has been removed from the installation even though the DNO supply is still connected leaving the installation as a TT system. Assuming the protection for the installation is correctly set up to be suitable for either supply this should not cause danger to the installation but would be a less effective means of protection than to use the DNO supply earthing.

In such a scenario, I would expect the EFLI to be lower via the generator than via the DNO supply.
Only ways I would see the EFLI being higher, is if there is no Neutral/Earth link at the generator and earth rods were installed, or if a Neutral/Earth resistor was installed.