ZS Values For Consumer Remote Consumer Units

You are confusing it all, staffs. The earth fault loop test you take at the db in the shed is recorded as the Zsdb. There is only one Ze and thats at the origin, irrespective of how many sub-mains there are coming off it. The Zsdb will comprise of the origin Ze plus the R1+R2 of the 6mm2 SWA. The Zs readings of the shed circuits will comprise of the Ze, Zsdb and the R1+R2 of each circuit. Zs is a live test, so it is whatever it is. R1+R2 values are dead tests and can be used to check the Zs.

For the shed circuits, Zs=Ze+(R1+R2 sub main + R1+R2 circuit)

 

As Unphased said essentially the Sub Board is a circuit it in its own right so Zs at the furthest circuit would consist of the Ze+(R1+R2 of the submain)+(R1+R2 of the final circuit) However Zs is not a live test it is a measurment of impedance. Calculating the the Zs Value by adding up the values of Ze and R1 R2 of the circuits will usually always give you a higher impedance then carrying out a live earth fault loop test and in many instances gives you a truer reading as you dont get parrallel paths as you would doing a live earth fault loop test.

 

Zs IS a live test. The circuit is energised when it is carried out. As I stated in my reply to staffs. R1+R2 is carried out dead and ZS is carried out live, there is no dead test for Zs. The mathematical check is Zs=Ze +(R1+R2) but as parallel earth paths often come in to play when doing Zs tests readings can often be lower than mathematically expected. But reading your post I think it is a typo as you go on to mention it being a live test.

 

Zs is the value of Earth Fault Loop Impedance, It is not a specific Live test. You can fill out the requiered Zs Values on a certificate without having to energise the circuits. It is simply the measured value of Ze or Value obtained form the distributor + R1 +R2

The measured value of Ze is always prefered for two reasons, A: its a more accurate reading then a generic expected maximum value, B: It actually confirms the presence of an earth connection.

You could take a direct measurment of Zs from a live installation but it is not a requirement or necessary.

 

I don't know why you think that Zs is not a specific live test? You can only measure it live or derive it from a combination of the Ze and R1+R2 values.  The formula is just a check, a guide. Zs is a requirement in BS7671 and is an accepted test. As I explained, R1+R2 is acquired in a circuit at the extremety for confirming that the earth path is in tact along the entire length. Thats the only reason you do it. You can add this value to the Ze but you are also required to do a live Zs test in compliance with the sequence of tests for a new circuit. Ze can only be measured live. A live loop test carried out at the end or furthest point on a circuit is the Zs. Are you recently qualified because you appear to misunderstand.

 

IF you measure R1+R2, which you should before making a circuit live, then there is no reason to measure Zs, it can be calculated.  On an EICR, where circuits are already live, measuring the Zs is quicker.

You measure your Ze at origin, then the R1+R2 of your submain.  This gives you your Zdb at the submain board.
On your next circuit schedule and test page (you have two, one for main CU, one for submain), you record the R1+R2 of the final circuit FROM submained board, the Zs for that circuit being the Zdb+the circuits R1+R2.

 

Because it isnt a specific live test.. Zs is just a value which can be caculated from the simple equation of Zs=Ze+(R1+R2) As you know.. The value of Zs can be measured live i dont disagree with that in anyway, the disagreement is with your understanding that the value of Zs when filling out a certificate can ONLY be obtained from doing a live earth fault loop test.. This is not the case, you can simply add your circuit continuity of R1+R2 to the external earth fault path Ze in order to abtain a true Zs for the circuit and use that figure on the certificate.

Im not sure how to make it any easier for you to grasp the fact that Zs is a value that can be derived from calculation or measurement and that both are acceptable when filling out the Zs value on a certificate and that when using the calculation method it will give you a true Zs value for that circuit in a "worst case scenario" as it gives a Zs value without and parallel earth paths which can give misleading Zs readings when carrying out a live Zs test.

 

Yes I understand. I think we were getting our wires crossed (scuse the pun). Zs can be measured, or can be derived, it doesn't matter.

 

Agreed UP although I always like to see a measured Zs, after all this gives an idea of the circuit integrity under actual working conditions. I never really feel that an R1-R2 test done with about 12 volts and negligible current (then a little bit of maths) can be as efficient as a nice few amps flowing during a "proper" Zs test!
I know the H & S brigade would ban us from doing any live testing of course if they had their way.

 

Majority of my Zs values are calculated on a domestic installs since the RCD regs came out

My Robin kit (which is still good as ever after all these years and sod replacing them!) will always throw these devices on testing.

 

SiFi
which rcd regs are you referring to ?
With robin DL 4120....solid stuff.
....... go up a range to avoid tripping ?
RS

 

You need a Fluke Sparky, you'd be able to do a "proper"  Zs test then without tripping the rcd!

 

My early Fluke 1652 is great at not tripping RCDs, but takes a long while to return te result.  Too long.  Testing with it set L to N, with N on earth is far quicker.  Instant even.  Obviously would trip RCDs then.

 

SiFi
which rcd regs are you referring to

The 17th RS, RCD's all the way on domestics

With robin DL 4120....solid stuff.

I have old yellow boxes mate, Multi test KT 1610 still going, Loopy tester and Insulation/continuity

....... go up a range to avoid tripping ?

At more cost for no reason, could always get a Part P tester!!

RS

 

Send us a cheque Sen, ill pick one up!

 

I had a KT1620, Si...unfortunately it was stolen out of my van. First tester I ever owned, purchased it 2003.