Ze=1.87 TT or TN-S?

[Bizron]

Morning all,

Looking for some opinions

MK Sentry Dual RCD 63/30 CU

Fluke 1662 readings...

2 lead hi current Ze= 1.87 Ohms PEFC = 6A PSCC = 0.774kA
3 Lead no trip Ze = 12.95 Ohms
Zs at cooker/sockets = 12.56
RCD times are compliant
No electrode visible and Gas & H2O 10 mm at board.

Encountered this on Friday during EICR. Reading is high for TN-S but great for a TT. I was assuming it was a TN-S arrangement on visual inspection only to be confounded and concerned by the reading. What to record it as is my problem.? Is that armour of supply cable acting as an electrode or not? Also if I need to contact DNO.

Cheers all

 

[diditrain]

It looks like you have 3 conductors on your supply cable, but who knows what is going on inside the cutout. You could try contacting your DNO to see if they have a record of the supply type to the property. ESQCR regulations require them to maintain the earth, if they are providing you with one.

 

[pc1966]

It certainly does not look right! Can you confirm that your 2 lead / 3 lead tests were both done at the same location? If you repeat the test do you get the same results (i.e. did a high current check alter the low current reading)?

While going from high to low current can lead to a loss of accuracy at low-Z it is not going to explain more than a fraction of an ohm, same for "RCD uplift" which is usually 0.5 ohm or so and probably not an issue with newer MFT equipment anyway.

My suspicion is a bad connection on the earth line at CU/MET/DNO but you need to rule out the customer-side of the system (CU & MET) as a problem.

 

[davesparks]

There's a dead giveaway that it's not a TT supply, the earth connected to the suppliers earth terminal.

And its quite obvious that there are three conductors coming in to the cutout.

So logically that is a TNS supply with a higher than acceptable loop impedance.

 

[Vortigern]

I would be very tempted to open the cover on the exterior box for more clues as to the layout in there. Then I would certainly discuss this with the DNO. A stab in the dark, it looks like the left most cable is the armoured outer twisted, going to an earth terminal and the other L - N therefore TN-S but can't really see from the photo. In any event it is clear the earth is not satisfactory and it is a matter for the DNO to correct.

 

[pc1966]

It certainly looks like exposed earth wire on the left-most DNO cable before it enters the cut-out, you could do a bond resistance check from CU to there, if it is small enough then the problem appears to be with the DNO.

 

[telectrix]

agree with all of above. looks like the cable coming up to the outside box is lead sheathed , the sheath being the TN-S earth. maybe a bad connection at that box or at cut-out.

can you see if you get similar readings at neighbours fed from same outside box?

 

[buzzlightyear]

That is a TNS came across one of those a a long time ago, and even looking at the out side the the big giver way is the solder on the outer cable.

 

[Strima]

99.99% that's TN, as has been suggested I would request a couple of quick tests done on neighbours properties to narrow down the problem and inform the DNO.

Take a look at page 24 of the attached, you could get the DNO out prety much straight away to sort this out.

MOCOPA-guide-version-3.5.pdf

 

[DefyG]

TN-S for me, It looks like a Split Concentric Cable has been used from the exterior box to the cutout.
Poor connection somewhere along the route!

 

[Bizron]

It looks like you have 3 conductors on your supply cable, but who knows what is going on inside the cutout. You could try contacting your DNO to see if they have a record of the supply type to the property. ESQCR regulations require them to maintain the earth, if they are providing you with one.

Thanks for that
[automerge]1590494777[/automerge]

agree with all of above. looks like the cable coming up to the outside box is lead sheathed , the sheath being the TN-S earth. maybe a bad connection at that box or at cut-out.

can you see if you get similar readings at neighbours fed from same outside box?

It certainly does not look right! Can you confirm that your 2 lead / 3 lead tests were both done at the same location? If you repeat the test do you get the same results (i.e. did a high current check alter the low current reading)?

While going from high to low current can lead to a loss of accuracy at low-Z it is not going to explain more than a fraction of an ohm, same for "RCD uplift" which is usually 0.5 ohm or so and probably not an issue with newer MFT equipment anyway.

My suspicion is a bad connection on the earth line at CU/MET/DNO but you need to rule out the customer-side of the system (CU & MET) as a problem.

yes all at same location main earth disconnected, terminations sound. Test done 3 times. My company does not allow any tampering with DNO kit so we’ll have to pass it on. Cheers
[automerge]1590495284[/automerge]

99.99% that's TN, as has been suggested I would request a couple of quick tests done on neighbours properties to narrow down the problem and inform the DNO.

Take a look at page 24 of the attached, you could get the DNO out prety much straight away to sort this out.

MOCOPA-guide-version-3.5.pdf

Thank you for that I’ll have a read of that later. I assumed it was TN-S as there are 3 cables and looks like the armour is the making the 3rd cable but the reading made me question it.

 

[DefyG]

I assumed it was TN-S as there are 3 cables and looks like the armour is the making the 3rd cable but the reading made me question it.

If it is a split concentric cable the earth conductors will be copper not steel as in swa.

 

[markythesparky]

Just out of interest, why are you doing a no trip test? Suppliers don’t give the consumer RCD’s.