PME or TT or Both To Caravan Or Outbuilding

Discuss PME or TT or Both To Caravan Or Outbuilding in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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Andy Barton

hi I'm a part qualified electrician gaining exp wherever I can whilst doing my college course.

Today we were hooking up a static caravan with 3 core swa. All was good until we started doing the tests. With the pme connected I was getting 0.26 as an earth loop at the board in the van. The boss then told us it really needed to be put on a tt so I duly banged in a stake and connected it all up. Now when tested is was getting just over 21 in my loop so I can't understand why this is better than using the pme.

I asked the question but neither the boss or the spark could really answer me. To add to my confusion I was also told to remove the core I had been using in the pme system to the met in the mobile home. I would have imagined belt and braces would be to have both earths connected the tt and the pme?

If somebody in the know could give me an answer on this it would be appreciated.

Thanks in advance
 
Pme not permitted on caravan sites supplying said caravans incase of open circuit neutrals on the supply making all extraneous parts in the caravan and possibly the caravan chassis live
 
Ok thanks I think I get that although it's not a caravan site but a farm with one static. Would the same logic apply to putting power into a container? However for a wooden or concrete construction you could take the pme over?
 
Sorry should have been clearer. So for one van you can take the pme over? Although surely the risk is the same for one van or having a field full of them struggling to get my head round the logic of this!
 
It is a static caravan I would treat it as any other building. Assuming the core used as a cpc can support the size of main protective bonding I see no problem. What was wrong with the 0.26 Zs at the caravan.
 
That's what is confusing me the 0.26Zs was great with the PME (one of the 3 cores being used as my cpc) it's the 20Zs when hooked up to the tt which worries me as you are binning a good cpc for the tt and also being told to lose the pme cpc completely in favour of the tt
 
Assuming the board is rcd protected then fault protection is provided. Have you asked why he told you to fit an electrode.
 
Yes board is rcd protected and checked out fine like I said couldn't really get a definitive answer as to why to use a tt possibly if the pme breaks down although I would have though more chance of a tt getting damaged. I can kind of get that but still can't fathom why remove the pme cpc completely and rely on the weaker tt cpc surely common sense would say if your going down that road leave them both connected??
 
Andy Barton said:
The boss then told us it really needed to be put on a tt so I duly banged in a stake and connected it all up. Now when tested is was getting just over 21 in my loop so I can't understand why this is better than using the pme.
Click to expand...

What kind of stake did you duly bang in? how long, how thick?
Did you test the Ra of this earth electrode or just trust to a loop impedance test?
 
A loop tester is an acceptable method for taking Ra or rather one of 3 methods accepted by GN3 tho an earth electrode tester maybe more accurate
 
Andy Barton said:
Yes board is rcd protected and checked out fine like I said couldn't really get a definitive answer as to why to use a tt possibly if the pme breaks down although I would have though more chance of a tt getting damaged. I can kind of get that but still can't fathom why remove the pme cpc completely and rely on the weaker tt cpc surely common sense would say if your going down that road leave them both connected??
Click to expand...
It is a hard wired static caravan not a caravan park.
 
Ian1981 said:
A loop tester is an acceptable method for taking Ra or rather one of 3 methods accepted by GN3 tho an earth electrode tester maybe more accurate
Click to expand...

I'm not sure that it is an acceptable method of measuring Ra, because it does not measure Ra, it measures the earth loop impedance including the earth electrode.

Either way I didn't suggest that it was not an acceptable test method, I asked if this method had been used or if a better method had been used.
 
Andy Barton said:
Yes board is rcd protected and checked out fine like I said couldn't really get a definitive answer as to why to use a tt possibly if the pme breaks down although I would have though more chance of a tt getting damaged. I can kind of get that but still can't fathom why remove the pme cpc completely and rely on the weaker tt cpc surely common sense would say if your going down that road leave them both connected??
Click to expand...

Common sense says you install it properly such that the TT method is not weaker or more susceptible to damage.

From your earlier posts I have got the impression that you have only installed a single Earth rod, in which case I would say that you have indeed installed an unreliable means of earthing and you would have been better off using the PME earth connection.
 
davesparks said:
I'm not sure that it is an acceptable method of measuring Ra, because it does not measure Ra, it measures the earth loop impedance including the earth electrode.

Either way I didn't suggest that it was not an acceptable test method, I asked if this method had been used or if a better method had been used.
Click to expand...
If he's made one end tt and disconnected the supply earth then yes he is measuring Ra

That's if he's disconnected it from the pme earthing
 
Ian1981 said:
If he's made one end tt and disconnected the supply earth then yes he is measuring Ra

That's if he's disconnected it from the pme earthing
Click to expand...

No, with a loop impedance tester he will be measuring the combination of Ra and the loop,impedance of the supply.
Ra is the resistance to earth of the earth electrode, it does not include the line conductor.
 
davesparks said:
No, with a loop impedance tester he will be measuring the combination of Ra and the loop,impedance of the supply.
Ra is the resistance to earth of the earth electrode, it does not include the line conductor.
Click to expand...
The earth fault loop impedance tester is connected between line conductor at the source of the installation (not in this case) and the earth electrode, and a test performed.
The impedance reading taken is treated as the electrode resistance.

Taken from GN3
So yes not as accurate as an electrode tester but is the method we can use to obtain Ra
 
Ian1981 said:
The earth fault loop impedance tester is connected between line conductor at the source of the installation (not in this case) and the earth electrode, and a test performed.
The impedance reading taken is treated as the electrode resistance.

Taken from GN3
So yes not as accurate as an electrode tester but is the method we can use to obtain Ra
Click to expand...

You are contradicting yourself.
You say it is treated as the electrode resistance, which is fine, but this does not make it Ra.
Ra is a defined quantity, the method you describe results in an approximation, not a measured value, of Ra.

You might as well be stating that g is 10 or that c is 300,000,000
Both are approximations and not the actual values.
 
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