Earth pin in a standard UK plug.

[tina-duncan]

Hi friends, simple question here.

I have a system that needs to be earthed, currently I am connecting it to an earthing rod which I have outside, however it would be more convenient for me to have the system indoors.

Would wiring the Earth connection to the Earth pin in a standard UK plug be a safe way of doing so (the remaining 2 pins would not be connected to anything).

Thanks,

Tina.

 

[Alan1000]

Hi friends, simple question here.

I have a system that needs to be earthed, currently I am connecting it to an earthing rod which I have outside, however it would be more convenient for me to have the system indoors.

Would wiring the Earth connection to the Earth pin in a standard UK plug be a safe way of doing so (the remaining 2 pins would not be connected to anything).

Thanks,

Tina.

Hi Tina,

Simply connecting directly to the Earth circuit is NOT a safe option.

If I understand your scenario correctly, I believe you would need to insert a resistor of (say) 1 mega ohm value between the system and the Earth pin. This will protect the system (and perhaps you) from electrical surges originating elsewhere in the building.

For example, if your system is in one office, and someone in the next office spills a cup of coffee into the photocopier, you could find 240V coming back up the Earth circuit towards you. You must have something to protect you from that.

Doubtless someone better qualified will soon give you a more informed answer. Until then, don't do it!

 

[AlephZero]

Simply connecting directly to the Earth circuit is NOT a safe option.

If that was true, virtually every piece of electrical equipment used in the UK would "not be safe", since except for "double insulated" devices, the metal case is always connected direct to the Earth pin.

Unlike wiring systems in some other countries, in the UK the Earth wire is indepedent of everything else. In domestic wiring, there is usually a direct connection from the Earth wire to the copper water pipes where they enter the house (before the main water shut-off valve) and the pipes then act as the "grounding rod".

So it is certainly safe to connect something direct to the Earth wire, though you didn't explain what you are trying to achieve by doing it.

 

[Alan1000]

Yes indeed... I know that there are differences between the UK and other countries, for example, the UK requires (if I am not mistaken?) that a fuse be incorporated into the plug, which is something not commonly required elsewhere.

From what you say, AlephZero, I would infer that the method of grounding is the same in Australia as it is in the UK; however, I do know that, in electronics workshops, the Australian WorkSafe Authority takes a dim view of direct-earth groundings which do not incorporate the resistor.

Of course, there are variables which differ for every site. Electricity will follow the path of least resistance. If that path does not include you, then conceivably a million volts will pass you by with no ill effect. I once saw a live 240V twin flex cut through with scissors, with no visible effect on the person using the scissors!

 

[Alan1000]

On reflection, I am not satisfied with my own reply. The variables are rather more complex than I first appreciated.

1. In the event that a short-circuit occurs elsewhere in the building, can we guarantee that Tina will always be electrically "upstream" from the short-circuit, regardless of where it occurs? If yes, the real danger to her is probably minimal, but I don't imagine for a moment that any local council or workplace safety authority would be happy to accept that situation as it stands.

2. Could Tina find herself located BETWEEN a potential short circuit and the ultimate point of grounding to the water pipe system? If yes, then she is in danger, because any unforeseen discharge will have to go through her (so to speak) before it reaches earth.

In my experience, the main distribution board is almost invariably mounted on an external wall and, in the case of older houses at least, quite possibly on the outside of the wall, exposed to the weather (although enclosed in metal box). If the distribution board shorts out (say, because rainwater is penetrating) then it doesn't matter where Tina moves within the building; she will ALWAYS be between the short and the ground point, and she will ALWAYS be in danger.

At this point, I would prefer not to contribute any more to this thread... it would be really great to see an electrical engineer, or someone equally competent, take over from here.

 

[AlephZero]

I have no knowledge of the Australian electricity supply system or its wiring regulations.

however, I do know that, in electronics workshops, the Australian WorkSafe Authority takes a dim view of direct-earth groundings which do not incorporate the resistor.

That sounds as it you are talking about earthing against static electricity damage to components, by wearing conductive wrist straps, etc. In that case, the resistor is to protect the operator if he/she accidentally touches a live circuit. It is not there to protect the equipment.

Earthing mains powered equipment through a 1 Mohm would be a potentially lethal thing to do, because if there is a fault in the equipment the Earth wire would be the highest resistance path in the circuit, not the lowest!

I can tell you what happens in the event of a "direct short circuit in a building" from personal experience, when an ancient electric oven I was using suddenly decided to short itself to earth. I was using a metal spoon to stir something in a metal pan on the cooker at the time. Apart from the surprise caused by the very loud bang, this had no effect on me whatever. The short instantly destroyed every fuse in the circuit right back to the distribution board - which is exactly what was supposed to happen.

 

[Studiot]

In domestic wiring, there is usually a direct connection from the Earth wire to the copper water pipes where they enter the house (before the main water shut-off valve) and the pipes then act as the "grounding rod".

I can agree with everything else AZ has said here, but this is a dangerous and sometimes illegal assumption.

The building electrical Earth has nothing whatsover to do with plumbing piping, which may be plastic, iron, lead, copper or whatever.
The regulations require earthing, even if there is no plumbing.

Unlike the USA, where the consumer provides her own earth, domestic customers in the UK normally receive their Earth connection via the metal sheath of the electricity supply cable.

It is a requirement that exposed metalwork in the building be 'bonded' (=connected) to this Earth connection if there is an electricity supply present.
The exact nature of this bonding depends upon circumstances.

One of the regulations requires that the cross sectional area of the Earth be sufficient for the fexpected fault current (defined by fusing arrangements) and in any event, not less than that of the phase conductor.

All of this is done in the name of protection - the UK earthing is a protective earth.

So Tina,

If you are earthing for protective reasons from power supply failure and using a building 13amp power supply then your scheme would be unconventional but satisfactory.

If you are earthing for greater protection then you will need uprated earthing arrangements.

But if you are earthing for signal purposes you will be in signal (not safety) trouble. Building Earth's are not suitable for this purpose.

 

[tina-duncan]

Thank you for your replies fellas!

I will elaborate on what it is I am doing, hopefully this will clear some things up.

I recently purchased an inverter which I have connected to a battery. As the inverter has a metal casing the manufacture states that it should be earthed, obviously to prevent shock, which is why I am asking if this is a feasible method of doing so?

 

[JaredJames]

Are you in the UK and if so does it have a three pin plug where the third (longest) pin is metal?

If that is the case then it is already earthed.

 

[Studiot]

Are you in the UK and if so does it have a three pin plug where the third (longest) pin is metal?

If that is the case then it is already earthed.

An invertor runs off batteries and may have no Earth connection.

You can purchase these for camping etc and run them from you car battery. I have one. Mine is double insulated and requires no earth. I carry it round on jobs in case I need a soldering iron.


Tina, you still haven't properly described your application or the apparatus location.

 

[Alan1000]

I have no knowledge of the Australian electricity supply system or its wiring regulations.


That sounds as it you are talking about earthing against static electricity damage to components, by wearing conductive wrist straps, etc. In that case, the resistor is to protect the operator if he/she accidentally touches a live circuit. It is not there to protect the equipment.

Earthing mains powered equipment through a 1 Mohm would be a potentially lethal thing to do, because if there is a fault in the equipment the Earth wire would be the highest resistance path in the circuit, not the lowest!

I can tell you what happens in the event of a "direct short circuit in a building" from personal experience, when an ancient electric oven I was using suddenly decided to short itself to earth. I was using a metal spoon to stir something in a metal pan on the cooker at the time. Apart from the surprise caused by the very loud bang, this had no effect on me whatever. The short instantly destroyed every fuse in the circuit right back to the distribution board - which is exactly what was supposed to happen.

Yes, now I understand! Thank you for that explanation, AlephZero.

 

[davenn]

From what you say, AlephZero, I would infer that the method of grounding is the same in Australia as it is in the UK; however, I do know that, in electronics workshops, the Australian WorkSafe Authority takes a dim view of direct-earth groundings which do not incorporate the resistor.

Being in Australia ... The above comment is ONLY applicable to anti static wrist straps
ALL other equip Earth connections are DIRECTLY connected to a real Earth via the mains power cabling back to the power switchboard.
In both Australia and New Zealand the Mains Neutral and the building Earth are bonded
together at the switchboard.

Dave