Zs values recorded results.

[fatboyslim]

Evening everyone,

I was taught when carrying out Zs testing to test both L-PE and L-N and record the highest result of the two tests for my Zs column.
However, speaking with a friend he informed me that this is wrong as Zs = Ze + (R1+R2) and not (R1 + RN) which is right and never crossed my mind.
All of my work is in-house commercial maintenance so when testing for Zs, L-PE is usually lower than L-N because of the parallel paths. Only on some circuits with a smaller Csa CPC, then the L-PE is higher.

Just wanted to know other peoples thoughts on the matter?? Am I recording the wrong results??
In the terms of safety, if the circuit max Zs value passes the highest of the two readings then happy days anyway.

 

[westward10]

Yes you are wrong, Zs is is the line to earth fault loop path.

 

[Julie.]

You say recording the values - why are you recording the values?

If it is to confirm ADS - this is particularly the earth path, so just lit Zs relates specifically to the earth path if you are confirming ADS, then you measure Zs and not the L - N path.

 

[fatboyslim]

You say recording the values - why are you recording the values?

If it is to confirm ADS - this is particularly the earth path, so just lit Zs relates specifically to the earth path if you are confirming ADS, then you measure Zs and not the L - N path.

I was taught to record the highest value of the loop tests so either L-PE or L-N for my Zs column. Which is clearly wrong!!

 

[Julie.]

Look at the regulations, Zs is defined as the earth loop path so essentially R1 + R2 (+ Re).

It has nothing to do with the neutral path.

ADS is always related to the earth route.

Actually there is no specific disconnection time within the regulations for line to neutral faults - only that the cables withstand the fault current for the fault duration.

The 0.4 sec etc is earth path specific!

It looks like whoever taught you didn't actually understand the regulations.

 

[pc1966]

For ADS what matters is Zs - the earth fault loop impedance (Ze + R1 + R2)

For checking if the OCPD can safely break a fault you should take the higher fault current of:

• PFC = fault to earth
• PSCC = fault L to N in single-phase, or the three-phase bolted fault current in 3P which is close enough to twice L-N PSCC

 

[pc1966]

Actually there is no specific disconnection time within the regulations for line to neutral faults - only that the cables withstand the fault current for the fault duration.

When I noticed that I was surprised! It seems simply wrong that you can have an acceptable system where you get a L-N fault and not even disconnect if no risk of fire, etc, but sure enough that is the case.

Of course it is an odd/pathological system arrangement where that is possible, but it is still possible to achieve.

Something I would try and avoid if possible by OCPD choice, even if not strictly required.

 

[fatboyslim]

It looks like whoever taught you didn't actually understand the regulations.

100% agree with that!!

 

[westward10]

I was taught to record the highest value of the loop tests so either L-PE or L-N for my Zs column. Which is clearly wrong!!

Who taught you this, are you confusing Zs with prospective fault currents.

 

[fatboyslim]

Who taught you this, are you confusing Zs with prospective fault currents.

Not confusing them, definitely was taught about recording the highest reading of L-PE or L-N as Zs. He's reasoning was about short circuit faults which as stated in a earlier post there is not a required disconnection time for short circuit faults.

 

[westward10]

Was this a teacher or work colleague.

 

[Julie.]

When I noticed that I was surprised! It seems simply wrong that you can have an acceptable system where you get a L-N fault and not even disconnect if no risk of fire, etc, but sure enough that is the case.

Of course it is an odd/pathological system arrangement where that is possible, but it is still possible to achieve.

Something I would try and avoid if possible by OCPD choice, even if not strictly required.

The reason is that ADS is all designed around protection of the individual in the event of contact (nail into a cable shorting to earth/cpc sort of thing) this determines the duration of fault (and to some extent the voltage - which should be less than 2/3 × 230V [due to the higher cross section of live conductors to cpc])

Hence why the time is specified.


There is still a requirement for proper design in terms of operation for L-N faults, it just doesn't have to meet the ADS times.

So yes, the OCPD must have sufficient breaking capacity for the maximum fault level, the cables/equipment must withstand the FLC for normal operation AND for fault conditions - that would be both line to earth (Zs) faults and line to neutral - although they could have different fault levels and duration to each other.