12v inverter to 240v mains - grounding?

[chris661]

Hi there ladies and gents,

I have to say, I don't know much about power supplies and the like, only enough to be dangerous. Since I'm planning on building something for a friend, I'd like to make sure that the final product isn't that dangerous.

What I'd like to do is run a small (couple of hundred watts, absolute maximum) PA setup from some 12v batteries. This will involve mics, DI boxes, and a mixing desk.
Now, I know I can use a battery and an inverter to get 240v mains. What I don't know is how I should go about grounding the mixing desk etc to ensure that the microphones aren't floating at 120v above ground, which I know from experience can give a jolt.

I haven't been able to find much online about this, so any advice would be very much appreciated.

Cheers
Chris

 

[JonSnell Electronic]

For all outside events using inverter power or generators, to comply with safety legislation, an earthing rod should be driven into the soil and connected to the earth/chassis of the equipment. Much in the same way as the earth wire in the plug does. It is acceptable to run a single core 2mm cable from the earth pin of a plugtop to the grounding spike that is planted in the soil. That will earth the mixer chassis and amplifiers.

 

[AndrewT]

the batteries are isolated from the Mains.
The load should also be isolated from the Mains.

You cannot get a mains electrical shock from ISOLATED equipment.

Adding a Main Earth to you isolated equipment increases the risk of a mains related electrical shock.

keep the whole system isolated.

Insulate all the high voltage wiring and high voltage terminations.
Consider double insulating where there are easily touchable terminations, or wiring that can be exposed to abuse.

This is not trying to build a ClassII product, you have no mains power.
This is not trying to build a ClassI product with a Protective Earth, you have no mains power.

BUT !!!!!!
if any of the connected equipment/load has the ability to be plugged into the Mains, then the whole system becomes capable of being mains connected. Then you must use Protective Earth on every part.

 

[DF96]

See this for the UK HSE guidance on this matter. A quick read suggest that an earth is not required (as post 2 claims) and not prohibited (as post 3 claims). It suggests that small systems may be best left ungrounded, but in any case whatever is done must be done properly.

 

[AndrewT]

I want to add that the reason Mains power is dangerous is that we stand on the ground and that ground is also the return route for current back to the Source.
Because we stand on the Return route it means that a "One Touch" can electrocute.

A system that is isolated from the mains cannot use the ground we stand on as the return route. If it is isolated, then it cannot share the return route. It is isolated from the mains return route.

An isolated electrical system REQUIRES a "TWO Touch" contact across the dangerous voltage to become fatal. Standing on the ground does not enter into it.

It is the difference between one touch and two touch that makes mains electricity more dangerous.
That is why we have so many regulations to protect us.

 

[chris661]

Hi gents,

None of the electrical stuff will be DIY - I'll be using a Behringer mixing desk, and an active speaker. All 3-pin IEC connections.

This setup will end up being used for small outdoor gigs, busking, that sort of thing. There's a fair chance of a concrete floor, so driving an earth rod is unlikely, though it looks like I won't need to.

Thank-you very much, all, for the help and advice. Next thing to do is size up some batteries.

Cheers
Chris