TT and exported earth

[Knobhead]

This has been bugging me for a while, why with a TT system shouldn’t the earth be exported? Surely the more earthed points the better, whether they be in the building or a remote building.
I’m not interested in the regs, but a sound reasoning.
To my mind if a SWA is earthed only at one end and the remote installation is soundly but independently earthed there’s a possibility of a voltage gradient between the armouring and the installation it serves. If the were connected it can not occure.

 

[kung]

Exported earth effects PME buddy. Its a long subject and if needed you'd convert the PME to TT in the remote building.

 

[kung]

A good guide is here
A good guide For 'exported PME' and outbuildings!

By IQ Electrical in forum Electrical Forum | General Electrical Forum

 

[Knobhead]

As I said:
"I’m not interested in the regs, but a sound reasoning".
Part of the reason I find this hard to grasp is because I’ve worked on HV systems where the dangers of ground voltage gradients are always present. It looks comical to see someone approaching a OH switch, placing one foot only half a step in front of the last.

 

[telectrix]

IMO, if you've got TT system and export this to an outbuilding, there is good reason to stake at the outbuilding as well. trhis will possibly improve the earth on the house installation as well.

 

[Knobhead]

IMO, if you've got TT system and export this to an outbuilding, there is good reason to stake at the outbuilding as well. trhis will possibly improve the earth on the house installation as well.

Exactly my reasoning.

To me it’s like an earth nest in an industrial situation. The more stakes and wider spread the better.

 

[Electro-tech]

If you are exporting the TT earth from a MET to another building then as said ^^^^you would need to stake it.I think that it is the connections and mechanical protection or lack of, could lead to a break in the earthing conductor. Perhaps we should run 2 earth conductors each with there own terminal at the Electrode & the MET?

 

[kung]

See your point now misread your thread nothing wrong with having another tt stake and cant argue with your reasoning there.

 

[NoSparks]

To my mind if a SWA is earthed only at one end and the remote installation is soundly but independently earthed there’s a possibility of a voltage gradient between the armouring and the installation it serves. If it were connected this can not occure.

Not quite. If there is a voltage gradient there is a voltage gradient. If the SWA is not connected then there is no current associated with the gradient (not through the cable anyway). If the SWA is connected there will be a current flowing through the armour of the cable, the armour is effectively bonding the remote earth just like a water pipe or gas pipe. Is this current advantageous in any way? Instinctively I would say no. The (safety) requirement is to provide an equipotential environment in the remote building. A current flowing in the armour will not assist this process. What could be the disadvantages? well the armour resistance will be low (probably less than 1 ohm) so even a very small voltage gradient of say 1 volt will cause 1 amp to flow there by heating the cable - hence reducing its usable capacity. By the time the voltage were worth worring about - say 50 volts - then 50 Amps flowing! (no I think other effects would come into play, but you get the point).

I'm sure I could go on rambling but maybe I should let others in at that point.

Thanks for making me think about it!
Geoff

 

[topquark]

I think there may be a terminology issue here. If you export TT you are generally not providing additional rods. If you provide the additional rods then you are extending the TT (not exporting it).

I don't see anything wrong with extending it to an outbuilding as long as you are providing additional rods.

 

[Knobhead]

The only time there would be a voltage gradient of any significance would be under fault conditions. The better the return path the quicker the fault clears and the lower the gradient. Does it matter if it’s going out via the stake at the house or at the shed?

A constant gradient and therefore constant current flow can only be created by galvanic conditions in the ground and to create that you’d have to put in two stakes of different metals. Zinc and iron in acidic soil would be a bad combination.

 

[topquark]

The only time there would be a voltage gradient of any significance would be under fault conditions. The better the return path the quicker the fault clears and the lower the gradient. Does it matter if it’s going out via the stake at the house or at the shed?

A constant gradient and therefore constant current flow can only be created by galvanic conditions in the ground and to create that you’d have to put in two stakes of different metals. Zinc and iron in acidic soil would be a bad combination.

Assuming that all of the earthing cable sizing is the same, almost certainly a marginally higher Zdb (maybe only a Zs depending on the circuit(s) arrangement in the shed) if you use a conductor as earth, if you use conductor and sheath then maybe slighly less of course. Given that it's TT probably not significant in the overall earthing impedance equation, assuming it's not a relatively long distance between them.

I guess there is an increased risk in loosing the physical earthing; unless it has been extended. Fault clearance time would depend whether there is any additional protection (ie RCD/RCBO) at the shed/source end, or possibly both ends (with some sensible discrimination).

 

[NoSparks]

The only time there would be a voltage gradient of any significance would be under fault conditions. The better the return path the quicker the fault clears and the lower the gradient.

Very true, but the fault that has to be considered is when a local factory has a fault in its nutral supply before the last PME connection. Then all their current flows back via earth creating a gradiant. This may persist for quite some time as it is not immediately obvious. Hence the reason for having 10mm bonding to incomming supplies.

Geoff

 

[truckster]

The OP was regarding a TT installation, the PME should not have any effect. If you are extending the TT then you may have a potential gradient should a fault occur in the extended earthing conductor but by rodding it at the remote installation this may well increase the potential. I would think it better to keep the two installations seperate earth wise.

 

[Deleted member 9648]

If you are exporting the TT earth from a MET to another building then as said ^^^^you would need to stake it.I think that it is the connections and mechanical protection or lack of, could lead to a break in the earthing conductor. Perhaps we should run 2 earth conductors each with there own terminal at the Electrode & the MET?

Cant see the logic here....if the earthing system (rod) is good enough for the main building why add another stake at the outbuilding if a cpc runs between them?...It's just an extension of the installation. If it's not good enough for the outbuilding it cant be good enough for the main building either.

 

[malcolmsanford]

In many situations in order to lower the value of Ra you can insert rods either one on top of another or inserting then in a cluster.

If you had a rod in building 1 and then 20 mtetres away had building 2 with another rod IMO opinion there is a case for making them 2 individual systems, or linking the rods to lower perhaps a high Ra.

With todays technology and the reliance of RCD as additional protection the Ra of a rod is perhaps not taken as critical, unlike when I first started, but I still like to achieve as low as possible Ra, and if that is achieved by linking the rods then I would be happy to do that.

In reality a PME system is just really a systems using several interlocked spikes in the neutral conductor................... not to dis-similar to a TT

 

[NoSparks]

I totally agree. The problem is where the TT installation is between a PME user with a fault and the source of supply. Then their fault will create a gradient across the TT installation premises. Having one earthing point for the TT system at each location means that location has one equipotential reference point.

At least that is my understanding of why we need bonding. (Ref diagam in Electrician of about August last year.)