Zs testing results query

[PetesHouse]

Hi just looked at a test result given on electrical installation cert to a friend for an install and wandered should alarm bells be ringing?

R1+R2 = 0.41 ohms
R2 = 0.21 ohms
Zs = 0.2 ohms

Now I thought Zs was obtained by Ze + (R1+R2)

So I don't know how they arrived at 0.2 for Zs am i missing something?

would it make any difference if it were TT or TN-S supply?

Any help to clarify greatly appreciated

Thanks

 

[ricicle]

Zs will almost always be lower than Ze + (R1+R2) due to parallel paths caused by main protective bonding etc

 

[JohnD]

"How they arrived at it" will be by using an EFLI tester which generates a momentary high current simulated fault, it is not done by calculation.

After replacing the main bonds recently in an old TNS installation, I found Zs had improved from 0.7 to 0.5 Ohms, presumably because the new bonds were continuous with lots of old lead and steel gas and water pipes in, around and under the house.

 

[PetesHouse]

Thanks for the reply
So its all down to parallel paths and EFLI testing

I understand that Zs can be determined by calculation Zs= Ze(R1+R2) where Ze is obtained by enquiry from the DNO so the questions poses how much variation is acceptable between this calculation and taking a direct reading using EFLI tester? Or does it not matter as long as its within the acceptable max range of 0.8 or 0.35 for TN-S and TN-S-C supply respectively. I dont know about TT supply but presume this would be substantially higher?

Thanks again

As can been seen always learning....

 

[JohnD]

Enquiry from DNO is only a guess, and usually includes a hefty dose of "worst-case" adjustment but will not include an actual breakdown. The chance of the guess being accurate is extremely low.

 

[westie101]

Or does it not matter as long as its within the acceptable max range of 0.8 or 0.35 for TN-S and TN-S-C supply respectively.

An interesting point there that, sadly, I've found a lot don't understand.
So some points

The figures quoted above are taken from a Electricity Supply Industry (ESI) document, they are not an imposed figure but one the ESI worked out some years ago.
The 0.35 figure for TN-C-S came about because of the additional parallel paths on a PME network.

The figure only has one purpose, to ensure that the DNO fuse will operate in a safe manner for faults up and to the consumer's fuses/mcbs.

From that as it is an ESI figure it means that we can work within it, on that vein it is permissible for a TN-C-S (PME) supply to use a figure of 0.8 safely as it will ensure correct operation of our 100A fuse.

If we find values above 0.8 as temporary measures we can reduce the fuse size, fit an RCD (or both) for very high values (where we have provided an earth terminal) we may also disconnect the supply until we can repair (in these cases we have 18 hours to do so).

There are no set values in the ESI for TT earthing as that is the customer's responsibility

 

[Spark123]

I understand that Zs can be determined by calculation Zs= Ze(R1+R2)

Careful, you need a + in there as what you typed actually means Ze multiplied by (R1+R2)

where Ze is obtained by enquiry from the DNO so the questions poses how much variation is acceptable between this calculation and taking a direct reading using EFLI tester?

If you ask the DNO they'll just give you the maximum figure. All you are doing is making sure your circuit will disconnect in time, with most smallish type B MCBs the max figure from the DNO will be OK as your circuit resistance won't be too high.

I dont know about TT supply but presume this would be substantially higher?

A TT system uses a consumers electrode as the return path for earthing. The Ze (RA) in this instance should not exceed 200ohms, RCDs need to be used to provide automatic disconnection as opposed to MCBs or fuses.

 

[PetesHouse]

Thanks for all the very informative replies

Careful, you need a + in there as what you typed actually means Ze multiplied by (R1+R2)

Well spotted Spark123 thanks for the correction

Just to further clarify in my mind hoping the coin will drop:
Zs maybe taken by direct measurement with EFLI tester with Earth and Bonding all connected, hence the parallel paths and lower result than Ze +(R1+R2) would give by calculation.

But if I was to measure Ze at the origin and then continuity R1+R2 and apply the calculation I would get a higher reading as parallel paths would have been omitted?

Does this mean direct measurement of Zs is the preferred and accepted method for testing and recording results on the cert or is that another can of worms?

Thanks again..

 

[1john]

It doesn't matter as if you calculate it you will be, nine times out of ten, recording a higher figure and as long as that figure is within the max permitted value of the protective device it's irrelevant.