Finding Zs the Hardway

AngelLaHash · Jan 9, 2024

Now Ive done Periodic Inspection and Initial Virification Qualifications with LCL/EAL (so non C&G 2394/95) #

[ElectriciansForums.net] Finding Zs the Hardway

strange part building Modules , this is like a part of the building than you slide in and then hook up.
my Manager says i could find out the Zs , i look puzzled at this but the guy with me was so excited.
As far as i look at it , i dont know where its being INSTALLED so dont know what type of system it will be TN_CS, etc
• 0.8 Ω for TN-S system.
• 0.35 Ω for a TN-C-S system.
• 21 Ω plus the resistance of the installation earth electrode for a TT system.

Then i also dont know the bit from the CU to the Module , dont know what switching will be put in

So how can you find out Zs when there is so much missing , i feel like some times managers lie about what they can and cant do to drive you on
Manager did scream at me saying that i couldnt work on a building site, i've worked on a few and im not agressive or pushy try to help all where i can so i guess a bigger building sites i may get ripped apart , but most i have worked on have been friendly to the point of looking after me. (I seem to have one of them looks that says this one need cotton wool around them)

buzzlightyear · Jan 9, 2024

Regulation 612.9 permits earth loop impedance to be determined by means other than measurement. Therefore, where reliable measured values are available for the external earth loop impedance (Ze) and for the loop resistance of the line and protective conductors (R₁ + R₂) of the circuit, it is permissible to derive the loop impedance of a circuit by using the following formula: Zs = Ze + (R₁ + R₂).

westward10 · Jan 9, 2024

Without knowing your Ze you cannot state a Zs, Ze being part of the path which Zs encompasses.

pc1966 · Jan 9, 2024

You can inquire of the supplier as to what the supply characteristics should be, but on old properties, etc, you might not get far with that. Also you really need to know the values for design work:

Highest Ze (thus Zs) for meeting ADS
Highest of PFC/PSCC (lowest impedance) for OCPD safe disconnection & adiabatic limits, etc.

Of course, once you are on site you should measure it as early as safe/practical to do so in order to confirm it is what you expected and so is not subject to faults or incorrect information.

Now some would argue that if you deign for a TT-like Ze, for example by using an all-RCBO board, then you are covering all cases so the actual values are not important, provided they are lower than 200 ohm (or as applicable from from Table 41.5 & Note 2).

However, I am not happy with that approach as doing a TT system is a bit more involved than that as you need to install for lower probably of faults before the RCBOs, etc, where supply earth fault disconnection won't happen on the OCPD (in most cases). Also it means your protection is down to the electronics forming the RCD side of the RCBOs, and that is far more complicated and likely to fail than the thermal-magnetic trip of the MCB side.

So while it is technically compliant as it meets ADS by design, I would rather not have the single point of failure in the RCBO's electronics (that realistically nobody tests outside of EICRs, and even then not always...) so would prefer to have something like a 100mA delay RCD up-front as a final line of defence if the RCBO fails.

AngelLaHash · Jan 19, 2024

pc1966 said:
You can inquire of the supplier as to what the supply characteristics should be, but on old properties, etc, you might not get far with that. Also you really need to know the values for design work:

Highest Ze (thus Zs) for meeting ADS

Highest of PFC/PSCC (lowest impedance) for OCPD safe disconnection & adiabatic limits, etc.

Of course, once you are on site you should measure it as early as safe/practical to do so in order to confirm it is what you expected and so is not subject to faults or incorrect information.

Now some would argue that if you deign for a TT-like Ze, for example by using an all-RCBO board, then you are covering all cases so the actual values are not important, provided they are lower than 200 ohm (or as applicable from from Table 41.5 & Note 2).

However, I am not happy with that approach as doing a TT system is a bit more involved than that as you need to install for lower probably of faults before the RCBOs, etc, where supply earth fault disconnection won't happen on the OCPD (in most cases). Also it means your protection is down to the electronics forming the RCD side of the RCBOs, and that is far more complicated and likely to fail than the thermal-magnetic trip of the MCB side.

So while it is technically compliant as it meets ADS by design, I would rather not have the single point of failure in the RCBO's electronics (that realistically nobody tests outside of EICRs, and even then not always...) so would prefer to have something like a 100mA delay RCD up-front as a final line of defence if the RCBO fails.

it would be a New build, at a guess a block of High end appartments
what i am guessing from doing Modular work .. a Modular Bathroom, and another department does Consumer Units and i think they are making a third lot for Kitchens (not 100% sure on that)

pc1966 · Jan 19, 2024

AngelLaHash said:
it would be a New build, at a guess a block of High end apartments

If it is a new build in any town or city with new incoming cable to the building it is pretty much certain it will be TN-C-S supply, (though obviously you should check with the DNO that is the case).

In that case your Ze value is essentially the supply impedance so the OSG value of 0.35 ohm is a good starting point for the design.

It might be lower of course, but as 0.35 ohms represents around 9% drop for 60A load that is likely the upper end of what the DNO would be happy to deliver. Note they have different rules to the 5% drop of wiring regs, and their cut-out fuse need not meet 5s disconnection as it is not for ADS against shock, but overload/fault protection on their network, etc.

AngelLaHash · Jan 19, 2024

so Ze 0.35Ohms and my Part of the Circuit just half of R1+R2 is 0.19353Ohms as its 1.5mm x 10m
so well under but dont know of any switch gear being put between the Module and the CU
Personally i think Zs would best suit the person installing the Module than myself building a part of it