Running power safely from gen set

[Emvee]

Hmmmm we shall see

 

[JohnW2]

Hmmmm we shall see

Maybe! The odd thing is that, on the basis of possibly flawed reasoning, I have posted a viewpoint which flies so much in the face with what appears to be 'conventional wisdom' regarding generator supplies that it seems very odd than no-one has jumped in to 'put me right'. To slightly modify what you say, we may see!

Kind Regards, John.

 

[holmslaw]

..

 

[JohnW2]

Hmmmm we shall see

... and I should have added, unless/until someone appears and give some constructive explanation as to why/how you and I are wrong, the natural assumption to make would be that we are right.

Kind Regards, John.

 

[stillp]

As this is on private land and not subject to buillding regs I don't need to worry about Part P compliance

Whatever gives you that idea?

The fact that this installation is not associated with "dwelling houses or flats" perhaps? Which means it falls outside the scope of part P.

Arguably it could be classed as a place of work, with all that that implies.

Ah yes. I thought he was saying Part P didn't apply because it's on private ground.

 

[ericmark]

Consider the simple situation in which a generator is to be used only for the local supply (say, to outbuildings) with no interaction with any mains-supplied installation. Also, in the first instance, assume that there is no earthing (i.e. no earth rod).

I am considering this which with Class II appliances I agree likely.

If the L&N outputs of the generator feed all the loads via an RCD and all loads are supplied with a CPC connected directly to the generator's neutral (not via RCD), then the RCD will operate in response to any L-CPC fault and (if the circuit is loaded) probably any N-CPC fault. If the generator (and circuit) are actually 'insulated' from earth (i.e. the whole supply is floating) then there is theoretically no risk to anyone who touches an L conductor, even if the person is in contact with true earth. As always, there is no protection for a person who touches both L&N, regardless of whether or not they are earthed.

Nearly correct however make up your mind either "a CPC connected directly to the generator's neutral" or "actually 'insulated' from earth" with the latter it's just like using a shaver socket and as long as there is only ever one item used then very low risk. However that's not the case as described he wants so hard wiring so two or more items may use the supply at the same time. This is where the problem starts.

Now introduce an unintentional (genny standing on soil, and its N connected to frame) or intentional (earth rod) connection from the generator's N to true earth. What is now different? The RCD will still operate in response to L-CPC and (usually) N-CPC faults. Unless the earth rod had an unusually low resistance, an L-E fault would not cause an OPD to operate. There is now a risk of electric shock to a person coming in contact with L whilst also in some continuity with true earth but, if the current through the person exceeds 30 mA, that should cause the RCD to operate in the normal way. If the resistance from the person to true earth and/or the resistance of the earth electrode is too high for 30 mA to flow through the person, then the RCD will not operate (just as no RCD would operate with <30 mA flowing though a person from L to E). There is still, of course, no protection for anyone who touches both L & N.

So what is actually gained by having the earthing, and why is the quality (lowness of resistance) of the earthing considered to be so important? In other words, what am I missing?

Kind Regards, John.

To me forgetting regulations for a moment. Should an item become faulty we want it to trip a protective device before it has a chance to make something live which could be touched at the same time as another thing made live by an independent fault but on a different live wire.

So if for example an electric drill left plugged in has something fall on it smashing it so connecting lets say a work bench to the neutral cable.

Now a hot air gun becomes faulty connecting the nozzle to line when this is put down it may touch some metal work which then becomes connected to line.

Without a bonded earth system neither will trip any RCD so touching both metal structures will give one a shock.

As I said it will take two independent faults this is why shaver sockets have single outputs and using the generator for one item is not a problem.

What we hope and of course there is no guarantee is that one of the faults due to an earth connection will trip the RCD before anyone gets a shock. But the RCD can't work without an alternative path so unless there is an earth system the RCD will not work.

Now I will admit in a building there is no real need for a true earth but just a bonded system so one of the line wires is connected to all items bonded. But as I said before all must be at the same voltage and if the generator has for example a centre tap for use with 110v and this results in a 55 - 0 - 165 split when switched to 220v then earthing one leg is no good.

Yes I know this should not happen. But it does. Even found a Honda like that and one would think the better know makes would be OK.

So the only way is to test and I would agree if the supply is never taken from the building then a loop impedance test would be OK. However one can not normally guarantee that no one uses an extension lead to work outside the building. So to protect one needs to measure the ground connection. For this the loop impedance tester is useless as there is no reference point.

We all know we have found houses which have the earth missing and has been missing since the water pipes were changed for plastic and no one has been injured. So yes one may get away without an earth for years. However it seems when it does go wrong it's often a child who seems to cop it. Hence why I fitted RCD protection on all circuits in my house. My son and daughter have both tripped them from time to time so I'm glad I did. Even I managed to saw through a cable going horizontally along the wall.

So if you were doing a risk assessment are you really saying the risk is within acceptable limits? I would think not!

 

[JohnW2]

Nearly correct however make up your mind either "a CPC connected directly to the generator's neutral" or "actually 'insulated' from earth"...

I don't understand - I was talking about a situation in which both were true - all CPCs of the installation connected to the generator's neutral and the entire installation insulated from earth (i.e.floating).

.... with the latter it's just like using a shaver socket and as long as there is only ever one item used then very low risk.

precisely my point.

However that's not the case as described he wants so hard wiring so two or more items may use the supply at the same time.

Again, I don't understand. There is nothing about the situation I was describing which precludes or excludes hard wiring and multiple loads.

Should an item become faulty we want it to trip a protective device before it has a chance to make something live which could be touched at the same time as another thing made live by an independent fault but on a different live wire.

Ideally, yes, but that's equally true with a conventional mains-supplied indoor installation, and such an installation will fail to achieve that in just the same fashion as you go on to describe for the genny case, unless a 'bonding maniac' has been at work ....

So if for example an electric drill left plugged in has something fall on it smashing it so connecting lets say a work bench to the neutral cable.

If that happened without any any L-N, L-CPC or N-CPC connection occuring, then no protective device would operate (in any installation) unless the workbench were earthed (or 'bonded to CPCs') - and, unless you were one of those 'interesting' occasional visitors to this forum, you probably would not have the workbench earthed or bonded 'just because it was metal'.

Now a hot air gun becomes faulty connecting the nozzle to line when this is put down it may touch some metal work which then becomes connected to line.

Again, true - but, again, unless there were some good reason for the metal in question to be earthed or bonded, no protective device would operate (in any installation).

Without a bonded earth system neither will trip any RCD so touching both metal structures will give one a shock.

True - but, above, unless one goes around bonding all metal objects (to the CPCs) 'just because they are metal' that is going to be true of any installation, whether supplied by mains or genny, and whether connected to true earth or not.

What we hope and of course there is no guarantee is that one of the faults due to an earth connection will trip the RCD before anyone gets a shock. But the RCD can't work without an alternative path so unless there is an earth system the RCD will not work.

As above, having 'an earth system' will only enable an RCD to operate in the situations you've described if all the bits of metal you postulate these two damaged appliances touching simultaneously were bonded to that earth system - and most such metal would not normally be bonded, with either mains or genny supply.

Nearly Now I will admit in a building there is no real need for a true earth but just a bonded system so one of the line wires is connected to all items bonded.

That's what I was talking about - all CPCs connected to the genny's neutral and any metal that was deemed to actually require it bonded to those CPCs. However,as above, that would not normally include 'all metal objects' (into contact with which damaged electrical items could come).

....However one can not normally guarantee that no one uses an extension lead to work outside the building. So to protect one needs to measure the ground connection.

As I said/asked before, what is the relevance of the impedance of the 'ground connection' if the (genny) supply is not earth-referenced (i.e.'floating')?

Kind Regards, John.

 

[westie101]

Interesting discussion, we use them a lot for emergency supplies so always have our earth available.
I'll ask one of our generator suppliers engineers next week what they do for stand alone installations.

 

[JohnW2]

Interesting discussion, we use them a lot for emergency supplies so always have our earth available.
I'll ask one of our generator suppliers engineers next week what they do for stand alone installations.

Thanks. That will be interesting to hear.

Everything that Eric has just written seems to really be about bonding - of the installation's CPCs to whatever metal is deemed to require bonding, and that re-ignites the discussion about whether or not one should bond 'everything metal' - like metal baths, workbenches and doorknobs, to name but a few! However, no matter how 'fanatically' one wants to bond, one can do that just as successfully with a floating genny supply as with a DNO supply. My questions and uncertainties are about the relevance (if any) of a connection to true earth for a generator supply. With DNO-supplied power, there is no escaping the fact that it is earth-referenced - so one has to design installations accordingly (unless one is prepared to install a masive isolating transformer!). However, with a totally 'self-contained' local generator supply, one appears to have a choice - and I have yet to understand what is to be gained (apart from a potentially increased risk of electric shock, in certain circmstances!) by connecting it to true earth.

Kind Regards, John.

 

[westie101]

Finally caught up with the guy I wanted to see.

Not as clear cut as it could be but he informs me that it is their customer's responsibility to decide and install whatever earthing they deem appropriate, in reality this usually consists of a single earth rod connected to the generator external earth terminal.
The job I am on at present has a 38kVA generator equipped welfare unit on site, this is housed in a steel container with general lighting and power. The only advice seen on the generator is to install the attached earth rod.

I suspect that a lot can depend on the generator controls as the types we see have very sophisticated electronic controls and RCDs. Older types can have just an on/off switch with fuses or a circuit breaker.

 

[JohnW2]

Finally caught up with the guy I wanted to see. Not as clear cut as it could be but he informs me that it is their customer's responsibility to decide and install whatever earthing they deem appropriate, in reality this usually consists of a single earth rod connected to the generator external earth terminal.

Thanks. However, as you will understand, that still really leaves my main question unanswered - i.e. as to what is achieved (apart, in some circulstances, from an increased risk of electric shock) by connecting a genny supply (which is totally separate from any other electrical installation) to true earth. Any ideas? Any answers, anyone?

Kind Regards, John.

 

[plugwash]

If you don't tie your system to real earth what do you think happens when a fault causes the "live" output from your generator to come into contact with something that IS connected to real earth (or comes directly into contact with the ground)? all the metalwork tied to your generators protective conductor becomes live with respect to real earth.

 

[westie101]

all the metalwork tied to your generators protective conductor becomes live with respect to real earth.

But where is the circuit to allow current to flow?

 

[JohnW2]

If you don't tie your system to real earth what do you think happens when a fault causes the "live" output from your generator to come into contact with something that IS connected to real earth (or comes directly into contact with the ground)? all the metalwork tied to your generators protective conductor becomes live with respect to real earth.

Fair enough, but that would only be a risk if the supply were exported (e.g. via an extension cable to power outdoor tools) outside of the equipotential zone (which should have been!) created within the building(s) supplied by the genny (e.g. the outbuildings mentioned at the start of this thread). Within that equipotential zone, I don't think that the situation you postulate could arise - at least, not without a protective device operating - since anything within the buildings which was, or might be, at 'real earth' potential (i.e.extraneous-conductive parts) will (should!) have been bonded to the installation's CPCs. Is that not the case?

In any event, wouldn't the RCD operate in your scenario (if someone earthed touched any of that metalwork/CPC), even if the supply had been exported into the garden?


Kind Regards, John.

 

[JohnW2]

all the metalwork tied to your generators protective conductor becomes live with respect to real earth.

But where is the circuit to allow current to flow?

As I understand it, he's talking about the situation in which the 'L' of a floating generator got accidentally connected to true earth, so that the 'N' (and CPCs) were then all at supply voltage above true earth. However, as I've responded, the RCD would then presumably operate if someone simultaneously touched those CPs etc and truth earth - and, in any event, the situation could not arise within a properly constituted equipotential zone.

Kind Regards, John.