Generator earthing

As above, you might not need short-circuit protection if the generator itself solves the disconnection problem - but if you can't rely on that, then 434.2.1 would normally limit the length of conductors without fault protection to 3m (in addition installing in a manner to reducing the risk of faults and fire and danger to persons to a minimum).


Oddly when you consider L-PE faults and protecting against electric shock, you can adopt double/reinforced insulation or equivalent for any length of conductor - so it seems a bit inconsistent not to have an equivalent option for short circuits.


434.3 (i) does provide another exception for generators, between the generator and its 'associated control panel' without an apparent limitation on length (with similar demands for reducing the risk of faults and fire and danger to persons to a minimum) - but is the switchboard really the generator's control panel?


If you've got a separate MCCB/RCD for the generator anyway, it is really a lot more effort to re-locate into its own enclosure closer to the generator?


   - Andy.

Almost right. Generator > 10 metres or so of cable( yet do be decided which type of cable) > mccb/rcd in switchboard > changeover switch. Yes there is no short circuit protection or earth fault protection to this cable, overload protection being provided by the downstream mccb

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.

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

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.

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)

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.

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.

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

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.