TT Supply, Testing Questions..

[TPES]

Practicing my testing and tested my first TT supply today..

It is an old farm house in the middle of no where.

I know that there is things about the install that is not upto regs.. like a BS 842 Voltage breaker for example, and the main earth needs a new copper rod and putting in a pit.

I have a few other questions about the tests i done...



1. I have been told that one of the methods for finding the resistance of earth electrode is to do a Ze test at the board, and this will also be the approx resistance of the earth electrode.. is this correct ??

2. Im not sure what i would put in for the "short circuit capacity kA" on the cert.. any ideas ??

3.I have done continuity of the ring main, I have an end to end reading of, L = 0.38, N = 0.38, and the CPC is 1.03 ????? this isnt 1.67 times greater than the liveconductors, what could cause this reading ?

4. This is a general question on any continuity test of any install.. On my assessment when im asked to do continuity of a ring final circuit will the assessor also want to see me do the L - N continuity readings and go round all sockets on the ring taking L - N readings, even though these readings are not recorded anywhere as only L - CPC (R1+R2) Will need to be recorded and shown on the test cert?

5. While testing all sockets on the ring final circuit for R1 - R2 I had a range of between 0.27 & 0.36, No spurs on the ring. Even though these reading are out of the 0.05 allowance between readings would you still class this as acceptable and write down your R1+R2 as 0.36 ?

6. There is a second CU just for storage heaters, how do you go about testing this? is this classed as a totally different inspection and will need a seperate set of test results all together?? also cant do some of the tests on this board as theres no voltage untill a certain time of the evening.

Comment on photos as you wish.. I know theres many upgrades needed.

 

[Guest123]

Hi Sid.

1. Correct

2. The BS number should be marked on the cut-out fuse. If it's BS 1361 type 2 then 33KA, if it's BS 88 then 16.5KA. Or do you mean the MCB's ??

3. you say it's an old farmhouse, older 2.5 T+E had a CPC of 1mm not 1.5mm this could be why.

4. No.

5. Could be a high resistance joint at one of the sockets, plug terminals loose etc.

6. Seperate Circuit details and Test Results sheets.

 

[Spudnik]

Agree with above, however, i would say that if the assessor asks to see continuity tests on the RFC, i would include the L-N test as well as the R1R2 and end to end's.

If he just wants an R1R2 he will ask for exactly that.

As for the NS CU, you can perform all your dead tests as usual and just calculate Zs, but make sure you note on the cert that that is how you arrived as these values.

 

[TPES]

Agree with above, however, i would say that if the assessor asks to see continuity tests on the RFC, i would include the L-N test as well as the R1R2 and end to end's.

If he just wants an R1R2 he will ask for exactly that.

As for the NS CU, you can perform all your dead tests as usual and just calculate Zs, but make sure you note on the cert that that is how you arrived as these values.

The second CU for the storage heaters has its own earth rod.. how will i get the PEFC & PSCC without voltage here? how would i get by this.. just cOme back late at night?

 

[Mr Mark Sparks]

if the 2 cus are from the same incoming main then i would say they should be on the same stake in this case i would see if it was poss to drop new earth from ns cu to other stake and base your readings on this if you do this check 1st reading to stake to prove its still the same

 

[TPES]

Where would you take your Ze readings from looking at the second photo attached?

I have assumed its the main tails and main earth coming into the top and terminated on the incoming of the BS 842 Voltage breaker.. Is this wrong ?

 

[Spudnik]

You need to get a true Ze so on the incoming of the RCD.

As Mark said, the Ze for the NS should be the same as the main CU, if its connected to the same rod.

 

[TPES]

You need to get a true Ze so on the incoming of the RCD.

As Mark said, the Ze for the NS should be the same as the main CU, if its connected to the same rod.

I did take my ze from the main incoming of RCD and got 0.27, still maybe i got something wrong, the second CU is not on the same rod.. its on a seperate rod.

 

[Spudnik]

You will have to excuse me here as i dont have the tester you are using but, are you sure you have it on the correct range and that it maybe should read 270?

Also, im not really sure why there is a separate rod for the NS.

Usually one rod, main earth to MEB and split into each CU from there.

 

[TPES]

You will have to excuse me here as i dont have the tester you are using but, are you sure you have it on the correct range and that it maybe should read 270?

Probally mate, my head has been all over the place today, It frustrates me so much..

Also it states in OSG the maximum Ze on a TT is 21 Ohms, Then it says anything over 200 Ohms maybe unstable. Im totally baffled as to what the Limits are.

 

[Spudnik]

General rule of thumb is that TT should be as low as possible but anything under 100 is fine.

 

[isitintheregs]

It is difficult to see from your pics but the lower cu has a link from its earth terminal to the one above. My guess from what you say is that the yellow/green from the lower goes to an earth rod as well as another one from the the upper cu to a different earth rod. In this case surely they are effectively the same assembly, possibly a second rod being put in to improve the first. That being the case the only concern as I see it is that one rod is a rods length away from the other.

 

[Des 56]

Probally mate, my head has been all over the place today, It frustrates me so much..

Also it states in OSG the maximum Ze on a TT is 21 Ohms, Then it says anything over 200 Ohms maybe unstable. Im totally baffled as to what the Limits are.

That 21 ohms is the suppliers maximum or stated, for the star point of the supply transformer and their own earth electrode
The maximum for the operation of a 30m/amp rcd is 1667 ohms however,

Any reading of your earth (which includes the suppliers 21ohms)should be under 200ohms

Anything above this figure could be considered as unstable,because for instance it may rise in drier whether when soil conditions are different

The 100ohms mentioned is an NIC figure erring on the side of extra safety and is guidance for their members only,the IEE regulations are the figures to follow for everyone else

 

[Spudnik]

Regardless of what the NIC say, i have always aimed to get it well below 100, even if it means installing 2 or more rods.

 

[Des 56]

The IEE regs require maximum of 200ohms
The figures any individual or the nic aim for or require, are a personal choice for them only

Im sorry but,
I prefer to inform what the requirements are rather than any personal options which may convey stricter results than are needed and may confuse rather than inform the questioner

regards Des


Obviously I fully agree the lower the reading the better and standards higher than the required standards are comendable