advice on Ze & Zs

[sparks1653]

Hi all,

i am currently training, and am just starting my level 3. i have started testing the pre-made training installations where there are intentional faults and resistors to simulate an actual installation, as well as watching others at my work carrying out testing of completed jobs. i am having a few issues with readings conforming to the given Zs in the regs.

for example in a house the Ze reading was 0.62, the R1+R2 for a ring main was 0.67ohms. when added together that value is higher than the given value of 1.16 ohms for a type b 32a breaker. the guy testing seemed to think it was all fine, i dont understand why?!

another one was a 6.0mm supply for a cooker, the earth conductor is so much smaller than the phase conductor it gives a really high R1+R2 reading, so i assume an R2 reading by itself would be the way to go there. but with a reading of 0.45ohms and again a Ze value of 0.62 it is over the given 0.82 ohms. again all seemed ok with the guy testing, and again i dont get why!

thanks,

 

[rodney12]

I take it the circuits mentioned are RCD protected? As if so the max Zs can be taken as 1667 ohms

 

[sparks1653]

yeah all the circuits are RCD protected. so any circuit in any csa of cable with any type of breaker would be taken as 1667 ohms so long as it is RCD protected? and if not the values in appx 2 of the on site guide apply?

 

[Electricalserv]

Never came across this 1667ohms rule can someone explain why?

Why not take a measurement if you are not happy with the calculation result?

 

[telectrix]

the 1667 is the value below which the RCD will trip.

measured value of Zs may well be lower than calculated due to parallel paths. and the figures in the osg are corrected assuming the cable temp. will go up to 70deg. i always enter the brb values on test sheets, but strive to get a reading below the osg corrected values.

 

[sparks1653]

so it is acceptable to have readings on a test sheet that are higher than in the OSG? what if you are testing a 3-phase board with no RCD protection, and the values are higher than in the OSG what is the correct way to deal with that? or is it ok to be over even then too.

 

[telectrix]

if the readings are above the max.. values, then either :

1. cable is too small for the job. or

2. poor connection/s pushing up reading. or

3. brb figures are too low, MCB manufacturers values are usually higher.or

4. it's too warm when you are measuring. if it's above 20deg. then resistance will be higher.

bear in mind that brb is not statutory and IMO a small percentage over and above the tabulated value is acceptable.

another point is that when measuring Zs, you are also measuring the resistance of the protective device/s. this can be ridiculously high.

 

[IQ Electrical]

It's a common 2391-10 question:

What steps can be taken if the measured value of Zs exceeds the corrected tabulated value?

1 Change the device type if the circuit/load characteristics allow it.

2 Reduce the length of run.

3 Use RCD device to meet disconnection times.

4 Install supplementary bonding.

5 Increase conductor CSA.

 

[sparks1653]

for example if you have a 2.5 mm ring main supplied from a type B 32a breaker and the Zs is too high, what method you presumably have no options for changing the device itself?

so what effects in terms of the permitted Zs readings what effects exactly do rcds have on circuits?

 

[telectrix]

be3cause the RCD will trip if the Zs is equal or lower han 1667, it's a back-up device, if you like. if the MCB doesn't trip in time because of high Zs, the RCD will. say, on a TT system, your Ze could be anywhere up to 100ohms, ( or 200 depending which book you read ) then the MCB cannot disconnect because the fault current is so low. thefore all TT systems must have RCD protection.

 

[IQ Electrical]

for example if you have a 2.5 mm ring main supplied from a type B 32a breaker and the Zs is too high, what method you presumably have no options for changing the device itself?

so what effects in terms of the permitted Zs readings what effects exactly do rcds have on circuits?

As Telectrix said, you use the RCD/RCBO to meet disconnection times where Zs is too high for disconnection times to be met by an overcurrent device.