Using generators with class 1 equipment - earth spike?

[Dave]

Should an earth spike be used when a generator is used to power class 1 equipment that normally needs an earth? (thinking of safety and ignoring EMC). I should know the answer to this but can see two sides of the coin and can't choose between the two. On one side it would be good to tie the gen and neutral down to Earth, but on the other side it would seem sensible to let the whole lot float. .. and how would an RCD cope? The particular case is using a gen to provide power for an outdoor PA system (and maybe a band).

Dave

[Mike]

Whatever you do, make sure all the mics and guitars have got their own personal in-line safety isolators.

Too many dead musicians in the past due to poor electrics - of course an area part pee doesn't cover at all.

[:::Jerry::::]

course an

But there was not need for Part P to cover this, other H&S regs cover it.

[Dave]

That's interesting, can you explain a little more. Do you mean isolating transformers? (or opto-isolators?) I google'd and came up with isolators to avoid earth loops but nothing about safety.

Dave

[BigWallop]

If it is a small 240 volts AC genset, then what do want an earth point on it for. If you're plugging directly in to the generator, then it already has, or better have, its own safety devices protecting it and the supply outlets.

A generator doesn't really need an earth point, and some generators don't really like being pinned to the earth, so you'd need to make sure that the metal work, if any, around the performance area was insulated from earth, but not the generator. Blocks of wood under the legs of any metal staging is usually enough to isolate the stage from earth.

If you're talking industrial generator here, then you'll need to follow the instructions for the generator you've bought / hired. These sometimes need to be spiked to earth for their own internal safety features to function properly. Others need to be bonded to surrounding conductive materials for the protective devices to work correctly.

It all depends on what and how the generator is going to used.

[Dave]

... snipped

It's a small 240 generator, there's an integral overload breaker but not an RCD. That's spurred a few more questions: - Why would a generator object to being grounded? - What is the metalwork on a generator connected to internally? - If the generator is floating why does any adjacent metalwork need to be isolated from earth? If it's truly floating there wouldn't be a path for current to flow.

Dave

[BigWallop]

Try a little test for me. With a voltage meter, place one test probe on the metal casing of the generator. Fix it tightly to the metal. With the other test probe, touch the Live terminal of the socket outlet. Make a note of the voltage reading it shows. Now touch the Neutral terminal and make a note of the voltage it has.

Now do a little bit of maths, simple addition, and add both the reading together. What do you get?

A generator doesn't have to have an earth point to ground, and some generators are more dangerous if they are fixed electrically to earth ground. It does depend on the generator though, and the in-built safety features it already has.

[Dan Mills]

I think he means RCDs.....

Usual practice for small gigs is a 30mA rcd either at the generator (which gives no discrimination), or several at thee distribution board. I often add a 10mA unit just for the backline because backline tends to be the root of all evil in this matter.

Do earth the generator frame with a spike, and DO bond the stage metalwork to generator earth (it probably will be via some path anyway and you would rather this didn't involve the screen of the audio snake....).

The spike is to protect you the generator operator in the event of an internal fault imposing voltage between the generator frame and the earth connection on the socket which is probably earthed elsewhere (like say the stage).

The RCD(s) protect the musicians from faults in the distro and their guitar amps (what never!).

Bonding the stage metalwork reduces possible earth loop impedance issues and encourages faster disconnect times.

Really once you get past the genny it is all fairly standard event power engineering.

HTH.

Regards, Dan.

[Dave Liquorice]

Yes, and bond the generator chassis to that spike and to which ever phase from the alternator you decide to call "neutral". ie replicate the sort of supply that the equipment is designed to be plugged into. A protective earth wire connected to real ground via a spike, neutral bonded to that earth at the supply source (generator).

Letting the neutral float is not good. One fault and that phase becomes as near as damn it earth potential but the other is now 240v away from earth. If that fault is in the live side the neutral is now sitting at 240v WRT earth. Single pole switches and/or single pole fuses, some thing looks off but is still very much live.

An RCD doesn't care about earths or lack of. All it cares about is that the same amount of current going to the load comes back (within a few tens of mA). If there is a current inbalance the difference must be going somewhere else which is probably A Bad Thing.

So put in a spike at the generator bond that to the generator chassis, bond a phase from the alternator to the chassis and earth spike and use an RCD.

[Andrew Gabriel]

You need to read the relevant section of BS7671 which I don't have to hand, and I don't remember that section off the top of my head, but I've guessed some answers below.

If the generator was supplying only one appliance, you could regard it as an isolated floating supply or an IT earthing system. However, as soon as you are driving multiple appliances (particularly something like an outdoor PA/band), it's not isolated anymore, and you probably need to make it a TT earthing system. In either case, the wiring regs still apply to the whole installation.

[Dan Mills]

Cite please?

I don't think I have EVER seen a general purpose generator which did NOT have either the star point OR the neutral end of the winding (if single phase) tied to both the ground terminal and the case!

Sure you can argue that an IT system is safer (but only if it has a earth fault detector) then a conventionally earthed system, but I don't think I have ever seen it done on an event site.

For any normal event power generator, drive a local earth stake and bond all the exposed metalwork to it (including the stage structure if appropriate) and make sure you use RCDs in the distro.

Regards, Dan.

[Mike]

Most guitarists will have one of these or similar already. Get something similar for any non radio handheld mics you provide.

of course you should also have RCDs in the supplies as other have mentioned.

[<me9]

We hired a generator from a reputable firm to plug into a fixed socket to supply fixed equipment via a changeover switch during a mains outage. The generator was 230 volts and had it's centre tap earthed, giving 115-0-115 output relative to earth. On the fixed wiring was a fault, not showing normally of an earthed neutral. This resulted in a lot of fried wiring. Without the earth spike/bonding it could have been fried operator. It is essential that for a temporary arrangement all safety checks are carried out

*and* earthing arrangements verified.

[bigcat]

truly, no, this is not a Good Idea (tm).

NT

[Dave Plowman (News)]

Often down to faulty instruments or PA systems. The common way these days is to PAT them all before use. Older method was to use a mains isolation transformer on each and every instrument. But this was in the days of valve amps where some didn't include their own internal isolating transformers. Most solid state gear will, as it runs off a lower voltage 'HT' rail.

We often have a 100 KVA genny out for filming where mains isn't available. And I don't *think* they sink an earth spike. They run a 45 amp (or two) circuit to a sort of rugged CU, and 15 amp circuits from that. I'm not actually sure what sort of breakers are in that CU, though. I'll ask on Monday.

[Dave Plowman (News)]

Is this the practice when you have a genny on location, Dave?

I'm pretty certain Lee Electrics don't do this with us, but not absolutely certain. But we have a genny out on Monday so I'll try and get chapter and verse from the operator.

[Dave Plowman (News)]

You're joking? Never ever seen this done. For a start, most decent mics are phantom powered, so need a DC path on screen and both signal lines.

Of course vocal mics are often non powered types, but I'd not be keen on introducing an extra transformer there. I'd say all mixers if CE approved etc will be perfectly safe if fed from a safe power supply, so I'd make that the route to follow.

[bigcat]

isolation

I'm guessing that the metal frame of a 100kVA gen would make good connection with earth without any spike.

NT

[Dave Plowman (News)]

Years ago when I worked on large OBs with gennys large enough to run several scanners with air-con, they *did* sink an earth rod. But export mains to remotes perhaps a mile away using a multicores - and joints - which had mains and several video and audio circuits all in one. Makes I laugh at the how do I get power to my shed type questions. ;-)

[Dave Liquorice]

Firstly this is for a single phase supply from a alternator with a single winding. Don't know about 3 phase, with star connected the central point is "neutral" and I think would be earth bonded at the source. Delta connected, I think there is still a "neutral" wire so like wise.

On a single winding alternator (ie two phases 180 deg apart) I'm

99.99% sure. Though the "earth spike" may just be laid on the tarmac and one of the wheels driven onto it... If there is a handy bit of open ground I have seen them hammered in but more often than not it's simply parked on.

Be interesting to know, I really don't like the idea of a floating neutral for the reasons I stated earlier. Ordinary equipment is not designed to be "safe" with the neutral floating, single pole switches and fuses in the live only etc.

I expect we'll be off a generator at St James's Park tommorow. If I can track down the operator I'll ask as well.