Standby generators and PME

I brief thought experiment involving connecting a generator frame and neutral output to an earthing stake (about 100ohm if lucky) and then opening the main service isolating switch to the house and connecting the house electrics to the generator leads to the possibility, with an external neutral fault in the supply system, the generator live being up to 750 peak wrt real earth. Is this a danger that should be prevented, and if so, how? It would seem to involve isolating the house protective earth from the utility supply while the generator was connected?
--

Roger Hayter

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On Sunday, 27 May 2018 13:07:31 UTC+1, Roger Hayter wrote:

The point of multi-point earthing is to stop the house earth connection going live. It does happen, but the risk is low and accepted.
240v rms is 330v pk so 660v pk on the live in the most exceptional circumstances. Not much of a risk, and dwarfed by what else would be going on.
NT
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On Sun, 27 May 2018 05:55:32 -0700, tabbypurr wrote:

If you must assume a worst case scenario, let's at least get the basic calculations right. :-)
Peak voltage for a perfect 240v rms sine wave (the mains is far from perfect[1] but let's put that aside for the moment), works out to be 339.40v. However, ignoring the new harmonised tolerance figures and stick with the original +/-10% figure (you never know), let's assume a maximum rms value of 264v and a perfect sine wave. This gives a maximum peak value of 373.333v for the mains supply.
Turning to the genset, if it's an old fashioned 'direct from the alternator output terminals' Dodo type with 'fancy' AVR control of the field winding current, I've seen these over-volt in response to modest capacitive loads to well north of the 270v mark (275 to 280 volts rms[2]) so to take a really conservative view, we need to assume a 280v rms maximum (for a nominal 230v rms output genset). Furthermore, we'd best assume a perfect sine wave to get 'worst case' figures, which gives a peak value in this case, of 395.960 volts. Let's round this up to 400v for a really pessimistic worst case value.
In the case of an inverter genset, typically calibrated to produce a precise and perfect 50Hz nominal 230v rms sine wave output (232 and 233 volts on a couple of the better version of Lidl's recent Parkside inverter genset offerings according to one of my plug in energy monitors) the peak voltage will be 329.50v, assuming it remains fault free. However, since the multi-pole three phase 'Y' connected flywheel alternator feeds 400v peaks into the 6 diode full wave bridge rectifier on the inverter module with its 450v rated 330µF smoothing cap to power the inverter module (effectively a pair of class D amps in bridge output mode), there remains a very small risk that an inverter module fault could result in voltage peaks of 400 to 450 volts appearing on the genset's output socket under no to very light loading, so whilst we're considering worst case scenario voltages, we'd better assume at the least, a peak of 400v as in the Dodo class generator case (preferably 450v).
Since, for the sake of safety, we assume a worse possible case for a 230v 50Hz inverter genset of 450v peak voltage, we may as well round this up to 500v maximum peak and whilst we're at it, do the same for the mains supply and round up not to a mere 400v but all the way to 500v. This then gives us a target isolating change over switch contacts peak voltage rating of 1KV or more.
In the overall scheme of switch contact voltage ratings, 1KV peak is not an awful lot more than the rather marginal 700v peak best case scenario rating. However, since switch contact voltage ratings are given in terms of RMS values, the above voltages correspond to 707 and 500v rms respectively. In practice, a 600v rms rated switch, with its own built in safety margin should suffice as a generator/utility transfer/isolating switch, especially if we assume a 400 and 450 volt peak for mains and genset respectively (a total peak value of 850 volts - 601v rms). :-)
When I was planning on diverting the two lighting circuits over to genset power, I was going to use a couple of standard 10A rated c/o light switches. Unfortunately (or perhaps fortunately in this case!), I ran into an insurmountable problem with the 2.8KVA PowerCraft Dodo genset I'd purchased from Aldi a decade ago[3] and the APC SmartUPS2000 I was using to feed my "Protected Supply" sockets scattered around the house, so never got as far as actually wiring anything up. Just as well since the ordinary light switch plates I'd bought were only rated for 250vac and these needed to have a 600vac rating for their intended purpose.
My plan had been to take a 3A fused feed from the genset to the two lighting circuit change over switches and power any other kit from the existing "Protected Supply" sockets (TV, fridge, freezer, CH mains feed as well as the existing IT kit already plugged in). This way, I'd save placing the lighting load onto the SmartUPS2000's protected supply. The genset was powerful enough to use it this way (2.5KVA cont./2.8KVA surge).
My current plan with the Lidl inverter genset is to now feed the lighting circuits (via 600vac rated change over switches) from the "Protected Supply" since it has a higher capability than the 1KVA inverter genset feeding it. Back then, some 8 or 9 years ago, I still had a lot of incandescent and CFL GLS lamps with not an LED to be seen since the more efficient than a CFL LED hadn't yet been invented/marketed.
Back then, even with most of the GLS lamps being CFLs, the total lighting load, all lights ablaze, would have been just shy of the 1200W mark. Today, including 140 watt's worth of 12v 35W downlighters in the shower room, it now only comes to 525W of which only some 150 to 180 watt's worth would typically be kept switched on each evening. It now looks like such a small inverter generator will keep the essentials, including a few essential luxuries, powered up during a protracted outage. It's a little bit marginal for my liking but it may prove good enough to ride out a severe outage with some hands on management to keep the chest freezer from defrosting.
With regard to the issue of earthing and dealing with floating inverter output terminals, I took the pragmatic approach (we're on a TNC setup that uses the steel armouring as the sub-station earth connection) by using a gutted plug in filter to bond the generator neutral and earth terminals to the house earth and neutral via a ring main socket adjacent to the one used to power the SmartUPS2000 so that it's merely a case of transferring the UPS plug from the adjacent mains socket into the adapted filter block socket. The live pin in the adapted filter block remains disconnected with the corresponding live pin socket connected to the live feed from the generator connection.
This does rather rely upon whatever caused the power outage not being a disconnected earth and neutral between the cut out and the sub-station connection whilst leaving the live intact, luckily a fairly improbable, if not altogether impossible, fault occurrence. :-)
As for that discrete earthing terminal on all portable generators, one excellent reason for using it with an independent earthing rod, even when earth resistance values of hundreds of ohms are the best you can hope for, is that it provides an anti-static earthing connection to reduce accidental ignition during fuelling operations with petrol/gasoline generators.
I can only recall seeing this specific mention of the anti-static earthing precaution once in all of the many user guides I've read in recent years. IME, the earthing pin(s) in the 13A socket(s) is(are) never cross bonded internally to the pin(s) designated as the neutral. It seems this is a decision left entirely to the end user's discretion. TBH, I'm not entirely sure whether the 13A socket earth pin is actually bonded to the generator chassis. I must test this sometime soon before relying upon assumption alone.
[1] I've observed (and I appear to not be alone in such observations) that the UK mains supply waveform approximates to a slightly flat topped sine wave where the flat top effect shows a slight negative down-slope on the positive peaks and vice versa the negative peaks. Consequently, this means that the peak voltage in practice is a little less than theory predicts.
[2] When the much vaunted "AVR" of a Dodo generator is completely overwhelmed by the effect of capacitive loading inducing 'wild excitation' into the rotor, the no load sine wave shape appears to be retained with the capacitor acting as a filter to mute the slot noise that typically pollutes the output waveform of such generator heads. I suspect that the actual peak value will be a better match to the theoretical peak value than in the mains voltage supply case.
[3] A whole decade ago! How time flies! No wonder it took me so long to track down a now ancient posting to alt.energy.homepower on the subject of generator/UPS compatibility issues way back in June 2010 just to confirm how far back it had been when I first embarked upon that ill fated generator UPS backup upgrade.
Now, at long last, I'm ready to start looking for a couple of 600vac rated 5 or 10 amp change-over switches to implement the lighting circuit transfer switching circuit. :-)
--
Johnny B Good

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On Monday, 28 May 2018 00:18:33 UTC+1, Johnny B Good wrote:

I don't

I did. The problem with your worst case calc is it's based on a string of highly improbably events all occurring at once. So improbable that I wonder if it's ever occurred in the UK. Which makes it fairly immaterial.
NT
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On 27/05/2018 13:07, Roger Hayter wrote:

If you have an external neutral loss on a PME them you have also lost the earth.
--
Adam

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On Sunday, 27 May 2018 15:23:21 UTC+1, ARW wrote:

With a local genny you add a local earth rod, so it's not that simple.
NT
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So in theory, especially if you have a fairly low impedance earth, then someone else's neutral return current can be trying to flow down it. In our case, sharing a pole transformer with 4 houses with overhead supply, there won't be that many (if any) multiple earths apart from the one at the transformer pole and mine. In theory the neutral for all of us could separate from both transformer and earth near this pole, leaving the neutral side of the transformer secondary earthed.
--

Roger Hayter

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