Generator earthing

Whatever cable you have prior to RCD or MCB, can only be mechanically protected - SWA will be fine, but my  point was that it does not have proper ADS - a short in that cable will only be stopped by the generator stalling the motor, or snapping a link (on PTO, not sure about that) or releasing the magic smoke from the windings...

You understand the NE link as being at the genset, as do I.

Mike.

Hi mike


Yes that was my concern and has always been with these type of generators, in my case tractor driven. You are basically supplies with an alternator and that’s it. Had it been a set with a control panel incorporated in it, then any mcb/rcd could be incorporated into it thus protecting the cable from there to the changeover arrangement. With my Mcb/rcd being located in a switchboard roughly a 10 metre cable run from the generator, then as you say there is no protection for this cable at all other than the tractor stopping or the pto shaft snapping!! At least with meter tails you have the supply fuse for overcurrent protection I suppose but no fault protection Which can be partly negated by double insulating and careful installation. As you say, swa should be ok for this, but would a suitably sized tough rubber flexible cable perhaps  be an alternative as it is essentially double insulated Between genset and switchboard whereas the swa won’t be?

You will struggle to find a 160A rated RCD. Better to put a 160A MCCB with earth fault protection close to the generator so that the cable from the generator is protected for both overload and earth faults. Blakely do MCCBs in enclosures with adjustable Vigi units that would be perfect for your job.

Hi Mike


Sorry I wasn’t describing it right. I have a 160a mccb in my switchboard and intend to have a vigi incorporated into It when my supplier gets it. I would then be content that the overcurrent protection for the cable and generator is provided by the mccb albeit downstream. The vigi will provide fault protection to anything downstream of it although any consumer units downstream already incorporate rcd’s so the vigi will be time delayed. My only concern remaining was fault protection to the cable between the generator and mccb/vigi. This is where I thought that a tough rubber flexible cable, although probably not classed As double insulated, may be better than swa which could potentially have metallic parts in terms of damaged outer sheathing exposing metallic armour or exposed glands. There would then be no fault protection for this eventuality

Actually, thinking further a  box with 3 fuse holders and some 200A fuses in it would at least provide "death or glory" protection.

Does the genset not have AVR (automatic voltage regulation) to wind the magnetic field up and down in the spinny bit to give a more or less constant output voltage with varying load and engine revs?

I do not think I have seen a full size one without a control box providing AVR and a "frequency good" indication  at least - what stops the voltage from going all over the place, and how do you know when you are near 50Hz  ?


Mike.

Mike


I haven’t got a photo unfortunately but try to imagine a bog standard alternator with a shaft sticking out the end. All it has is a voltmeter and a frequency meter. There is a small circuit board inside the alternator which I assume is an avr.

OK, that sounds like the sort of thing - sitting  the cab and looking over your shoulder at the frequency indicator set the revs on the locking throttle to get 50Hz and end up with perhaps 400-500RPM on the PTO,  as required by the gearbox on the alternator. The ones I have seen can be floor standing or fixed to the 3 point hitch to allow things like electric weld repairs to broken farm machinery in the middle of a field (do not ask about earthing or wiring regs then)

Assuming the tractor is powerful enough, >100 HP, then you could probably blow a 200A fuse with this arrangement sometime in the future. Again we seem not to appreciate that the current available from Generators is not like a mains supply, it is strictly limited both mechanically and electrically. Overload is not the major problem with this kind of installation, the alternator could probably not melt a 70mm cable whatever you did in the way of drive power. The windings are forced air-cooled and really quite thin, but overload will usually slow the motive power significantly, and this will usually trip the AVR and so remove the output almost completely long before you damage anything. If you have enough engine to spin an 80kVA alternator at full speed at say twice full loading (meaning something like 220HP, you need a JCB modern fastrack) you might then pop a 200A fuse. In fact, you are likely to damage the drive gearbox first going from 540/1000 RPM to 1500 for the alternator. Remember too that the design is such that producing the correct output voltage is fine but making 1-2 volts at several hundred amps from the windings is another thing altogether, as there is a huge magnetic field problem with a very low load impedance and the reactance will go through the roof, also limiting the current. You do not need an overload fuse / CB, and too much load will simply not work as you imagine. Reading your last post, you do have an alternator with the correct ratio gearbox don't you, because if not you have no electrical problems at all! Note too that a tractor will only produce maximum power at the rated PTO speed, it needs to be right and is always marked on the rev counter.

Thanks for all your replies


I would like the idea of locating my mccb/rcd arrangement at the generator itself which would give me more peace of mind that the cable from generator to switchboard is protected against overcurrent and earth fault. However it seems in reality quite a lot of work to protect a few metres of cable considering that my mbbb/rcd is already incorporated in a switchboard basically beside it. Again thanks for your advice, I’ll weigh up the pros and con up over a beer tonight before I decide the course of action!!!


cheers

I'm not sure I've pictured it all correctly, but I think you're suggesting gen -> 10m cable -> change-over-switch -> main incomer in DB - with possibly no adequate protection from shock or faults until past the incomer?

   - Andy.