Earth fault loop impedance understanding

[Bronek536]

Hi all.

I hope someone can help.

I'm studying for Inspection and testing qualifications and stuck on understanding measured values of earth fault loop impedance. I hope I correctly interpret the formula Zs=Ze+(R1+R2).

Here is the value I measure at the origin between line and pe I read 0.24 ohm, at a socket outlet I measure 0.19 ohm. Shouldn't the reading has taken further away of origin be of greater value because of additional wiring?

Many thanks 

 

[Murdoch]

What was the Ze?

was there bonding in place when you did your Zs reading

dont worry if the difference is so small ... it’s really rare for Zs to equal  Ze + (R1 + R2) in the real world , especially if you are using socket adaptors 

 

[Sharpend]

Not sure I agree there Murdoch, when I used to do new builds I could get the zs within 0.02 of calculated but this was With plastic water and gas pipes coming into the property so no strong parallel paths to lose much. 
so I guess what I am saying is the tolerance of measured against calculated is dependable upon several factors. Parallel paths, loose terminations, poor socket connection if measured with socket lead. Etc. 

 

[Murdoch]

Not sure I agree there Murdoch, when I used to do new builds I could get the zs within 0.02 of calculated but this was With plastic water and gas pipes coming into the property so no strong parallel paths to lose much. 
so I guess what I am saying is the tolerance of measured against calculated is dependable upon several factors. Parallel paths, loose terminations, poor socket connection if measured with socket lead. Etc. 

aren’t you agreeing with me?

 

[Sharpend]

Yes but no but elaborating? 

 

[Bronek536]

Thank you for your rapid responses. I believe that I need a greater understanding of the fault loop path concept because from what I read I assumed we measure the fastest path via a circuit protective conductor to MET,  earthing conductor, for a TN system the metallic return path, the path through the neutral point of the transformer, the transformer winding and the line conductor from the source to the point of fault. With all the stages being listed I don't understand how is the fault current taking parallel paths in radial socket circuit for example after MET. If you guys can kindly refer me good educational resources on the subject I will be grateful. Back to the main point, to fulfil the formula by measurement do I need to disconnect all parallel paths from MET and take a reading, am I correct?

 

[boltonsparky]

When parallel paths are involved your fault loop path will also include the general mass of earth via any extraneous conductive parts. Disconnect parallel paths (bonding) for ZE test, reinstate parallel paths for PFC and Zs readings. 

 

[SPECIAL LOCATION]

Hi all.

I hope someone can help.

I'm studying for Inspection and testing qualifications and stuck on understanding measured values of earth fault loop impedance. I hope I correctly interpret the formula Zs=Ze+(R1+R2).

Here is the value I measure at the origin between line and pe I read 0.24 ohm, at a socket outlet I measure 0.19 ohm. Shouldn't the reading has taken further away of origin be of greater value because of additional wiring?

Many thanks 

Your thoughts are correct....

But also you have to allow for how good the connection is between your meter probes/clips and the cable you are testing...

Think about the numbers you are getting, also think about what values you get if you measure continuity on just your meter leads before they have been zeroed.

AND.. vitally important you need to keep an awareness of what values you are expecting to read, proportional to the cable conductor sizes..

e.g. If a reading taken a  couple of meters up a circuit is similar to, (or less than), a reading taken at the origin, that can be just due to the connection of your meter leads..

But if you were getting a lower reading say 60m away up the circuit then something may not be right?

your values of 0.24 and 0.19 is a difference of 0.05..

If this 0.05 was part of a 2.5/1.5 T&E radial..

then 0.05 = approx 2.6m of cable!!  

Have a look at the resistance values for copper cables in Appendix I of the OSG..

and consider if your differences equate to a gnats whisker of cable or a significant length..

Guinness  

 

[Bronek536]

I'm glad so many people willing to help here

Just elaborating further shouldn't the formula for Zs be taking the form Zs=Ze+(R1+R2)-R2p where R2p- resistance of parallel paths to means of the earth. Since introducing parallel paths in real word reduces the measured value of Zs. Logically I'm imagining some of the fault currents taking the path to means of the earth via bonding hence understandably measured impedance of loop path to the transformer because of some current going the other way to be reduced. 

 

[SPECIAL LOCATION]

I'm glad so many people willing to help here

Just elaborating further shouldn't the formula for Zs be taking the form Zs=Ze+(R1+R2)-R2p where R2p- resistance of parallel paths to means of the earth. Since introducing parallel paths in real word reduces the measured value of Zs. Logically I'm imagining some of the fault currents taking the path to means of the earth via bonding hence understandably measured impedance of loop path to the transformer because of some current going the other way to be reduced. 

Stop and think again  what are you verifying the Zs reading for??

i.e.   You are designing or altering a circuit.. 

Key objective is to make sure that in the event of an earth fault the MCB will trip fast enough to prevent danger.

This being verified by compliance with the Max Zs given in BS7671 tables (or manufactures data). 

So you design a circuit with a supply  ->  A protective device  ->  A length of cable  ->  A load.

You don't specifically 'design-in' any parallel earth paths, that may or may not crop up later due other equipment that gets connected to the circuit..

AND..  parallel paths may change over time due to alterations by other non-electrical related services..

(Customer has the heating system replaced / an old water tank and immersion heater are taken out / some pipes in the property are changed to plastic etc..)

You must prove the integrity of the  "Electrical installation" part of the earth loop path to be good, sound and intact.

So providing your new/amended circuit has an acceptable Zs by measurement or calculation of (R1+R2) and Ze,

then you can sign your certificate to say the circuit complies with BS7671.

If you based your Zs around some parallel paths that 4 months later are disconnected, you could be leaving a potentially dangerous installation.

But..

If you have a Zs verified by Ze+(R1+R1), the installation remains safe even if someone comes and adds a whole bunch of parallel earth paths two days after you've been there doing your testing.

Bottom line is Zs verified for all circuits, appropriate protective devices, and correct bonding your installation should fail-safe...

and you can sign your bit of paper with confidence.      

Guinness

 

[Bronek536]

Stop and think again  what are you verifying the Zs reading for??

i.e.   You are designing or altering a circuit.. 

Key objective is to make sure that in the event of an earth fault the MCB will trip fast enough to prevent danger.

This being verified by compliance with the Max Zs given in BS7671 tables (or manufactures data). 

So you design a circuit with a supply  ->  A protective device  ->  A length of cable  ->  A load.

You don't specifically 'design-in' any parallel earth paths, that may or may not crop up later due other equipment that gets connected to the circuit..

AND..  parallel paths may change over time due to alterations by other non-electrical related services..

(Customer has the heating system replaced / an old water tank and immersion heater are taken out / some pipes in the property are changed to plastic etc..)

You must prove the integrity of the  "Electrical installation" part of the earth loop path to be good, sound and intact.

So providing your new/amended circuit has an acceptable Zs by measurement or calculation of (R1+R2) and Ze,

then you can sign your certificate to say the circuit complies with BS7671.

If you based your Zs around some parallel paths that 4 months later are disconnected, you could be leaving a potentially dangerous installation.

But..

If you have a Zs verified by Ze+(R1+R1), the installation remains safe even if someone comes and adds a whole bunch of parallel earth paths two days after you've been there doing your testing.

Bottom line is Zs verified for all circuits, appropriate protective devices, and correct bonding your installation should fail-safe...

and you can sign your bit of paper with confidence.      

Guinness

Thank you so much for your time and help.