Appliance isolator switches

[bernardgreen]

So in real terms we can expect up to 20 volts earth electrode to true earth.

That can, in theory, also happen with TN-C-S. However, in either case I'm not sure that (within reason) the voltage between the installation's 'earth' (i.e. CPCs) and true earth is necessarily all that important, provided that everything is correctly bonded to the MET. The inside of the building can be an "equipotential zone" regardless of what that "equipotential" is relative to true earth.

Kind Regards, John

With very low impedances the current due to a 10 volt difference can be hazardous. with 10 volts and a 0.1 ohm impedance the current is 100 amps, with no fuses in the neutral or "earth conductor" or bonds to limit that current, if most of that 0.1 ohms is concentrated at a weak point then most of the 1000 watts being disipated ( 10 volts times 100 amps ) will be concentrated at that weak point.

 

[ericmark]

I have had one call where a radio ham had installed an earth mat for the transmitter, 4 earth rods with tape between them, the electrician would wired his shed (shack) decided he should bond the copper bar and ran a 4 mm² earth wire under the feed cable back to the house earth block. House on TN-C-S supply.

Owner had seen insulation melting on earth wire so turned off all power to house. However this did not stop the wiring heating up when I got there it was balls of copper on the ground. The DNO said when phoned there was no way the neutral/earth could be lost, but were there in minutes. Seems some workmen hit the cable. Every other house had equipment burnt out except the radio ham who had turned off his power.

He never did renew cable shed became a TT supply. In industry I have seen massive resistors between earth on site and DNO earth to stop this happening. Mind you on that site I spent 3 months putting in earth electrodes.

The TN-C-S system is that good it's not allowed for garages. I had to get a reading of 8Ω for every rod. But in a home 60Ω is a more normal reading, in fact if I see as I have many times readings of less than 8Ω when I re-test I have always found the person has taken reading without disconnecting other items so the water or gas pipes have given the good reading not the earth electrode.

 

[JohnHarrison]

Another in having RCBO isolators for heavy appliances is that only one ring need to installed, instead of one for the kitchen carrying all the heavy appliance and one for the rest. The one ring will only take light loads.

Thanks and.....

 

[JohnHarrison]

,,,

 

[JohnW2]

.... I'm not sure that (within reason) the voltage between the installation's 'earth' (i.e. CPCs) and true earth is necessarily all that important, provided that everything is correctly bonded to the MET. The inside of the building can be an "equipotential zone" regardless of what that "equipotential" is relative to true earth.

With very low impedances the current due to a 10 volt difference can be hazardous. with 10 volts and a 0.1 ohm impedance the current is 100 amps ...

I'm not sure what, in context, you are talking about - 10V between what and what, and very low impedance of what? Eric was talking about a possible p.d. (of 20V, IIRC) between a TT electrode and true earth due to leakage currents, which I can't see is usually going to be of any particular relevance/importance. The pd he was talking about was the consequence of <30 mA of aggregate leakage current flowing through a (relatively high impedance) TT electrode, not 100A !!What am I missing?

Kind Regards, John

 

[ban-all-sheds]

Another in having RCBO isolators for heavy appliances is that only one ring need to installed, instead of one for the kitchen carrying all the heavy appliance and one for the rest. The one ring will only take light loads.

So on the one hand you're happy to have half-a-dozen or so circuits for half-a-dozen or so appliances, and on the other you'll have one socket circuit for the entire house.

Way to go.

 

[JohnHarrison]

Another in having RCBO isolators for heavy appliances is that only one ring need to installed, instead of one for the kitchen carrying all the heavy appliance and one for the rest. The one ring will only take light loads.

So on the one hand you're happy to have half-a-dozen or so circuits for half-a-dozen or so appliances, and on the other you'll have one socket circuit for the entire house.

Way to go.

You got it. !

In my case:

1. The integrated appliances have full isolation.
2. Freezer contents are protected from faults of other appliances that trip.
3. Boiler is protected from faults of other appliances.
   
4. Easy to fault find.
5. One appliance does not affect another in a fault situation.
6. Faults are easy to isolate.
7. No ugly fused spur switches over worktops.
8. Isolators all in one place and labelled.
9. Only one ring need be installed as it will take only light loads.
10. Takes heavy appliances off the ring.

All positive and sensible.

Thanks and...............

 

[bernardgreen]

I'm not sure what, in context, you are talking about - 10V between what and what, and very low impedance of what? Eric was talking about a possible p.d. (of 20V, IIRC) between a TT electrode and true earth due to leakage currents, which I can't see is usually going to be of any particular relevance/importance. The pd he was talking about was the consequence of <30 mA of aggregate leakage current flowing through a (relatively high impedance) TT electrode, not 100A !!What am I missing?

Suppose the incoming Neutral is at 20 volts above Ground ( true earth ) potential. This means that the CPC and all items bonded to the MET will be at 20 volts above Ground potential.

If any "Earthed" ( via CPC ) or bonded ( to MET ) item is also in contact with the Ground then current will flow from the MET to Ground.

If the impedance to Ground is low then a high current will flow to Ground. If the bonded item is a metallic water supply pipe then the impedance to Ground could be less than 0.1 of an Ohm With the Neutral at 20 volts above Ground potential 200 Amps will be flowing to Ground via the bond wire to the water pipe.

This mechansim has caused fires when the Neutral was affected by cable theft or other faults have occurred in the local network.

 

[bernardgreen]

All positive and sensible.

All positive is not good

You need the negative flip flopping back and forth with the positive to make things work

 

[JohnW2]

Suppose the incoming Neutral is at 20 volts above Ground ( true earth ) potential. This means that the CPC and all items bonded to the MET will be at 20 volts above Ground potential. If any "Earthed" ( via CPC ) or bonded ( to MET ) item is also in contact with the Ground then current will flow from the MET to Ground. ... If the impedance to Ground is low then a high current will flow to Ground. If the bonded item is a metallic water supply pipe then the impedance to Ground could be less than 0.1 of an Ohm With the Neutral at 20 volts above Ground potential 200 Amps will be flowing to Ground via the bond wire to the water pipe.

We know all that, but it's got absolutely nothing to do with what eric was talking about. He was saying that, in a TT installation with 12 RCBOs, the accumulated L-E leakages of all connected equipment might (very theoretically!) be, say, 240mA (20mA x 12), without any RCBO operating, and if the TT earth electrode had an impedance of around 83Ω, this would raise the potential of the earth electrode to about 20V above true earth potential. I remain of the view that, even if that very 'extreme' situation were to arise, it would be of little/no consequence. If a very low impedance extraneous-c-p were bonded to that TT earth, the postulated total leakage current would not rise to noticeably above the 240 mA.

Kind Regards, John