earthing

[williamwright]

I'm building a self-contained semi-portable device.

It will perform the following functions: Provide 300W of true sine 235VAC power Illuminate a room during a power cut, or provide a work lamp where there's no mains Charge USB devices Provide compressed air, particularly in order to inflate tyres Provide a 30A 12V DC supply for short periods.

Components include:

22Ah deep discharge battery 300W inverter 12V 200W compressor USB psu (from 12VDC) LED lamp on ball and socket 2A intelligent charger Fuse block for low current 12V circuits 30A main battery fuse 25A compressor fuse 25A inverter fuse IEC mains male for charger input 13A socket for inverter output

What to do about grounding? I plan to connect the following together: The earth terminal on the IEC The earth terminal on the 13A socket The metal cradle that holds the compressor The earth terminal on the inverter The metal panel that holds all the controls (this will ground all the accessible metalwork such as switch bodies and toggles.)

Questions:

1. Is the above OK?
2. Should I ground the negative side of the 12V circuitry or let it float? It will not be grounded via the charger or inverter.

Bill

[No Name]

I'd probably treat this as if it was a "Honda Generator" and stick an earth rod into the ground, bonded to the chassis, I woudl also consider replacing fuses with RCBOs (quicker trip time and improved earth leakage detection) and bond neutral to earth before all RCBOs.

ALso I'd design out any scanerios where you are using a Jesus lead, especially in the dark. (best done with a Changeover switch like this one)

:-)

S.

[Andy Burns]

at 300W I can't see Bill back-powering the house from this portable marvel ...

[Harry Bloomfield, Esq.]

williamwright presented the following explanation :

Far too small to have useful capacity, driving all of those things. Did you really mean 220Ah?

[Dave Liquorice]

<SNIP>

Any interlock to stop the inverter and compressor running at the same time? Nominal 500 W is 40+ A @ 12 V...

Fused in the mains side somewhere other than the plug top?

Seems to bond all the metal chassis bits together and to the PE. Except that the PE won't be there unless the thing is plugged in. The only mains powered thing is the 2 A charger does that require a PE or is is double insulated?

Define "ground"? Are you refering to the mains PE that may or may not be present or just the bonded exposed metal work that is floating relative to real muddy earth?

I think I would bond the 12 V neg to the chassis. Just so everything is tied to the same reference. There are some hefty currents flowing and it doesn't take much resistance, 0.04 ohms to drop 1 V @ 25 A, mind you that would also be dissipating 25 W.

Debateable for the invertor mains out, which is effectively bi-phase floating. ie like running kit off an isolating transformer. Touch one side and that gets "safely" pulled down to earth.

er, the fuses are in the 12 V distribution from the battery to kit.

eh?

[Dave Liquorice]

Should get over an hour of the compressor or maybe 45 mins of the inverter flat out. Though the inverter will be pulling 30 odd amps, which may reduce the avaialble capacity of the battery.

The numbers of amps flowing for 12 V makes me wonder if going up to

24 V and thus halving them wouldn't be a bad idea. My 750 VA UPS has 2 x 12 V SLAS, the 1.5 kVA has 4 x 12 V SLAs...

I should imagine 24 V compressors and inverters etc are available from truck stores.

[williamwright]

No, but it won't happen. The battery fuse might be OK as a 25A actually.

No. 3A fuse in the plug top.

It's double insulated. The charger will only be used when the thing is indoors on a shelf. I won't even take the IEC lead with the thing when it is used. You'll have gathered that I haven't thought of a name for the thing yet.

Yes.

or just the bonded exposed metal work that is floating

OK.

Bill

[williamwright]

The biggest current draw is the compressor when it's working hard. It pulls about 18A but will inflate a flat (reads zero psi) car tyre to

35psi in five minutes. That's less than 2Ah even allowing for faffing about.

The inverter will only be used for an additional light or a DAB radio or whatever, and not for long periods.

The USB charger draws about 1A for a short time then tapers off.

The floodlight is a 20W LED, so draws about 2A.

The device has to be portable and I couldn't carry a heavier battery. To be honest I'm shocked at the weight even with a 22A battery fitted.

Bill

[williamwright]

Yes they are but I needed a 12VDC output to be available, so I thought, bugger it, do it at 12V. I have a 24V vehicle and in many ways would have preferred to use 24V.

Bill

[Chris Green]

A 25A fuse will take quite a long time to blow at 40A.

[Fredxx]

<snip>

Which is more reason to go for a Lithium battery.

A lead-acid battery is pretty poor at providing currents > C where the Ah is can easily be halved at just 1C discharge rate:

A lithium tends not to suffer the same and you can extract far more of it's power. It's also lighter for the same capacity.

[Brian Gaff (Sofa]

Do you have any positive Earth devices like old car radios where the case is grounded. One has to be really careful not to weld ones device to the nearest earthedbit of metal before the fuse goes!

Brian

[Nick Cat]

If it's double insulated you don't need to connect anything to mains earth. With things like this it's generally best to keep things as electrically apart as possible, rather than bonding it all together.

NT

[williamwright]

Yes. I might fit a 20A actually, see if it's OK.

Bill

[williamwright]

My worry is that since it's a charger I don't see how it can be truly double insulated. What if an internal fault allows mains to get into the output?

Bill

[Nick Cat]

no challenge there

Sounds like you're not familar with the principles of so-called double insulation.

NT

[williamwright]

Actually I'm very familiar with Sod's Law. How can you be 100% certain that the output wires of a charger will never carry any voltage from the supply? Damp? Circuit damage due to a lightning strike on the mains or other cause of a surge? Poor design? Faulty assembly? As far as I'm concerned any wires that come out of a box might for all I know make contact at some point in time inside the said box with any other wires that go into the same box. I also look left and right when the traffic lights are on red, and at open level crossings. And I hold my abdomen when I fart. And refuse 'mushrooms' picked by walkers. And I don't leave the electric kettle on its base because the switch inside the kettle might break.

Bill

[Adam Funk]

What's the safety issue with that last precaution?

[Nick Cat]

Yes, but not the principles of double insulation

immaterial really. No scheme provides that level of safety.

also immaterial :)

NT

[williamwright]

Well there you are then.

Bill