What's the difference between Zt, Zs and (R1+R2)

[duncanhead]

What's the difference between Zt, Zs and (R1+R2), I thought that Zs was the loop impedance, Zt ? and either R1 or R2 can be measured directly or derived from the difference between Zs and Ze, or am I totally confused?!

 

[Spark123]

Zt not sure on that one - possibly you have misheard someone say Ze?
Ze is the loop impedance external to the installation.
Zs is the overall loop impedance of the circuit.
You should not use Zs - Ze = (R1+R2) to calculate (R1+R2) as the (R1+R2) may include parallel paths.
However Zs = Ze + (R1+R2) is fine as if you measure the (R1+R2) omitting parallel paths using a low resistance iohm meter, then add it to Ze it gives a worse case scenario should parallel paths are introduced.

 

[duncanhead]

Zt not sure on that one - possibly you have misheard someone say Ze?
Ze is the loop impedance external to the installation.
Zs is the overall loop impedance of the circuit.
You should not use Zs - Ze = (R1+R2) to calculate (R1+R2) as the (R1+R2) may include parallel paths.
However Zs = Ze + (R1+R2) is fine as if you measure the (R1+R2) omitting parallel paths using a low resistance iohm meter, then add it to Ze it gives a worse case scenario should parallel paths are introduced.

yes it's a radial or light circuit, Zs is "automatically" measured using my Megger 1552, and I've measured Ze, both are consistant, Zt apears on Form "WR4" - Installation Schedule (including test results) and is described as Earth loop impedance, I think it's a typo and should be labelled Zs

 

[ericmark]

So am I what is Zt?

Zs - Ze approx = R1+R2 but since Z = impedance and R = resistance any inductance or capacitance in the circuit can mean they don't quite equal each other.

Also parallel paths can also result in different readings.

To work out the likely Zs of a circuit before power has been applied we have to use R1 + R2 + given Ze with a TN-C-S supply taken as 0.35Ω this allows us to see if the circuit will be of a low enough value to operate the safety devices.

Once power has been supplied we can't measure R1 + R2 so then we use the direct measured Zs value.

Again this will tell us if the protection device is likely to operate.

There is also the let through value of the fuse or PFC (Prospective Fault Current) to consider and the impedance of line - neutral at incomer and device can also be used to calculate volt drop. Or before power is connected R1 + Rn

 

[Spark123]

Switch your MFT to ohms and measure the R1+R2 which you should have done as the first test. (continuity of protective conductors).
Add this to Ze to give you Zs
Confirm Zs by measurement after it has been energized.
Which forms are you looking at? NICEIC??

 

[ericmark]

Sorry you posted supply while I was writing. The Installation Schedule only requires one method you do not need to enter both Zs and R1 + R2 they are different methods of getting same information.

 

[Spark123]

The schedules of test results as published by the IET have separate columns for (R1+R2) and Zs, they also ask for Ze at origin at the top.
If Zs cannot be measured for any reason then I may choose to use Zs = Ze + (R1+R2)
You can do away with the (R1+R2) if you want to go for the 50v rule and measure R2 using the wander lead method, never done that before.

 

[duncanhead]

Switch your MFT to ohms and measure the R1+R2 which you should have done as the first test. (continuity of protective conductors).
Add this to Ze to give you Zs
Confirm Zs by measurement after it has been energized.
Which forms are you looking at? NICEIC??

yes, done all that, Ze = 0.4 Ohms, Zs is about 0.57 Ohms, am measuring R1+R2 and also R2 in stages as and when I can isolate circuits etc

The form came off a "mega Part P" CD from Ebay...

 

[duncanhead]

The schedules of test results as published by the IET have separate columns for (R1+R2) and Zs, they also ask for Ze at origin at the top.
If Zs cannot be measured for any reason then I may choose to use Zs = Ze + (R1+R2)
You can do away with the (R1+R2) if you want to go for the 50v rule and measure R2 using the wander lead method, never done that before.

am in the process of measuring R2 using the wanda lead method as can do live, R1+R2 measurements will have to be done when all isolated later next week..

 

[Spark123]

You don't need to bother with R2 for circuits if you have measured R1+R2 and added to Ze give a result of less than the maximum measured earth fault loop impedances given in the On Site Guide or Guidance Note 3.
You still need your wander lead for your main protective equipotential bonding.
Could save you a few ££s too : http://www.theiet.org/publishing/wiring-regulations/forms/formspdf6.cfm?type=pdf

 

[duncanhead]

You don't need to bother with R2 for circuits if you have measured R1+R2 and added to Ze give a result of less than the maximum measured earth fault loop impedances given in the On Site Guide or Guidance Note 3.
Could save you a few ££s : http://www.theiet.org/publishing/wiring-regulations/forms/formspdf6.cfm?type=pdf[/QUOTE]
I think that I do need to measure for R2 as it dosen't exist on 1/2 of all lighting circuits (that's what was the norm in 1963)

 

[Spark123]

How are you going to measure R2 on a circuit without a CPC?

 

[duncanhead]

How are you going to measure R2 on a circuit without a CPC?

I can't - it's going down as a departure... told that's Ok as long as I state it...

 

[Spark123]

Yep, there is some info on the subject of replacing a consumer unit with a lighting circuit without CPCs here: http://www.elecsa.org.uk/downloads/...ing Circuits with no Protective Conductor.pdf

 

[duncanhead]

Yep, there is some info on the subject of replacing a consumer unit with a lighting circuit without CPCs here: http://www.elecsa.org.uk/downloads/...uits with no Protective Conductor.pdf[/QUOTE]

thx , thats very enlightening, but a mockery as it ultimately states "do not proceed with the replacement CCU" surely a newer split load RCD/RCBO at the correct rating is MUCH MUCH better than an older fuseable CU, or is Part P "an ASS"!!!!

(or I can just do it and not advise that its finally completed, it might take me 20 years then !!! - )