Earth Rod Needed?

[ericmark]

As said in the main equipment in the garden is class II so no earth, items used outside which are larger, narrow boat, caravan for example TN-C-S supply is banned, electric cars can have a special arrangement where if the voltage is not between 207 and 253 volt it auto disconnects first lives then earth, this is an odd one out, not seen on anything but EV charging.

The problem is two earths at a diffrent voltage, and the distance between them so the gradient of voltage, to use a caravan fire regulations means at least 2.7 meters from a building, so gradent likely shallow enough, but in storage my father-in-laws caravan was less than a meter from the gas and electric meter, and on concrete, so used the TN-C-S supply as could touch both electric meter and caravan together.

A radio ham friend has his shack around 4 meters from house, and the electrician who wired it used a 4 mm earth wire between the two, and bonded it to the radio earth bar, which in turn connected to 4 earth rods one each corner of garden and earth tape between them.

When road works broke the PEN, he noted plastic melting off 4 mm earth wire, so turned off all power, this made no difference as earth not through isolator so the 4 mm earth wire reduced to copper balls, the whole estate was trying to earth through his earth.

Ohms law means 40Ω earth will at 400 volt carry 10 amp, so a 40Ω earth electrode and wire must carry 10 amp to be safe, so you need the measure the ELI of the lamp post, and work out the prospective short circuit current, it is not a simple earth or don't earth, connecting the wrong side of a gas supply electrical isolator block to earth has caused fires, normally blamed on copper theft, but as with most accidents more than one cause.

So depends how close to other earthed items, I would normally use TT supply, but depends where it is, there are always exceptions.

 

[bernardgreen]

the whole estate was trying to earth through his earth.

being pedantic.... the whole estate was sending it's Neutral current back to the substation's Neutral terminal via the Ground and the Ground electrode at the substation which is connected to the substation's Neutral terminal.

 

[ericmark]

being pedantic.... the whole estate was sending it's Neutral current back to the substation's Neutral terminal via the Ground and the Ground electrode at the substation which is connected to the substation's Neutral terminal.

He was only person in the estate not to loose equipment, but the whole question of TN-C-S has been raised with EV charging points, and to my mind raising the voltage from 50 volts to 70 volts to earth is just sweeping it all under the carpet.

The whole question seems to be revolving around how many times TN-C-S has caused danger. It seems some are trying to say it goes wrong so few times we can ignore it, But with EV charge points this is likely to increase, but if the alternative is TT, then it is the question how often does a RCD fail to work compared with loss of PEN, and I can't answer that.

 

[bernardgreen]

I no longer have the notes but recall that there were several times at my previous house when the potential difference between a Ground electrode and TN-C-S CPC was a cause for concern. Not real problem as except when I was accurately measuring the CPC to Ground potential the CPC was "bonded" to the Ground electrode which was several feet of rod into the bed of a stream. Very low impedance. A current meter on the bond conductor did indicate significant currents flowing to Ground.

A cable fault between between sub station and the group of houses solidly shorted two phase conductors together under the High Street, The quick fix was to restore service was to connect those two conductors to the same phase at the substation. The resulting load unbalance created a lot of current into the local connections to Ground. My Ground electrode took a lot of the Neutral current. Another house had a lead water main which had been the original source of safety Earthing. When the house was re-wired the lead water main was "bonded" to the MET to preserve the equipotential zone but this bonding connection also provided a low impedance to Ground for the current from the bouncing Neutral ( via the CPC )

 

[ericmark]

As you say 20 amp through the bonding conductor is not wanted, but neither is it a problem, but enough current to melt conductor is, so unless measured you don't know.

 

[Robin0577]

The SWA terminates at a outdoor socket then from that goes to a weatherproof FCU then comes out in rubber flex (in conduit) as its a cast iron lamp post that was originally gas powered so there were no provisions for electrical cables

This would be my main point of concern... IF the lamp post does have the iron tail buried in the ground, and therefore a reasonably low impedance earth path, then the 0.75-1.5mm? flex conductor from the outdoor socket to the lamp is woefully inadequate as a bonding conductor, and could easily melt as Ericmark described in his post about the radio amateur. If the post is in contact with true earth, then surely it should be bonded to the installation earth with something rather more substantial, like 10mm? This might be achievable using the cable armour, depending on its copper equivalent size.

Or, it may be possible to avoid the whole issue of exporting the earth from the house by replacing the lampholder with a plastic one and the flex with 2 core if the insulation requirements can be met. Or install a SELV LED light source in the lamp post with the transformer remote from it.

 

[JohnD]

"no provisions for electrical cables"

A cast iron lamp post designed for gas, has an entry point at the bottom for the gas pipe, and passage all the way to the top, so there is no difficulty in running cables

The gas lamps in Farting Lane, next to the Savoy hotel, have another gas passage

 

[bernardgreen]

Or, it may be possible to avoid the whole issue of exporting the earth from the house by replacing the lampholder with a plastic one and the flex with 2 core if the insulation requirements can be met. Or install a SELV LED light source in the lamp post with the transformer remote from it.

Or treat the lamp post to a TT supply. with an RCD in the house

 

[ericmark]

Or treat the lamp post to a TT supply. with an RCD in the house

I would in the main go for TT, but the distance between it an TN-C-S is the main factor, I would say at least 3 meters between them.

 

[OxiaUK]

there is not straight answer here lol, the base of the post is sat direct in soil (no concrete) the flex supplying it has a 3a fuse via a spur and the whole thing has a 30ma rcbo in the house.

Does it need a rod? if so what size earth to the post and do I need to remove the earth that goes to it from the house?

I would have thought that the post was already acting as its own earth rod if it was in the ground and not in concrete?

 

[FrodoOne]

Hi

We have an old converted cast iron lamp post in our garden that has a copper head on and a 230v bulb... this is all connected via an SWA cable from the house protected by a 30ma RCBO.

Is this OK to be connected to the house earth supply (PME) or should it have a Earth rod ?

thanks

May I suggest that you sit down and view (for an hour or so )

Yes, it is long and "biased" towards US practices.
However it discusses the practicalities of "Earthing" - which US Citizens term "Grounding".