What’s the point of earth loop if your rcd performs ?

[Handyapprentice4]

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Hi guys was having a conversation as a apprentice I’m still not sure and was wondering why earth fault loop impedance is important if you’ve proved your earth because the rcd trips proving your rcd performs ? Can anyone tell me thanks so what my question is in short what’s the point of it if your rcd performs ?

 

[Dustydazzler]

RCDs can fail , correct continuity of earth is important

 

[Lucien Nunes]

This is often a bone of contention. If you mean why is it important to achieve specific low figures, it is to avoid relying on the RCD when ADS is a possibility as it is more reliable.

in the more general case, if the loop imepdance is abnormally high, it can indicate a faulty connection, inadequate rod etc. I.e. it's a symptom that leads you to a possible.problem even if it's not dangerous at that time.

 

[Dustydazzler]

America and Canada don't test EFLI or the Ohms to Ground but they do check for continuity of earth

I guess it could be argued that we don't need to fully test EFLI and record the Ohm reading just use one of those plug in Martindale type socket testers and if you get 3 green lights you are good to go ?

 

[davesparks]

if you’ve proved your earth because the rcd trips proving your rcd performs ?

Proving that an RCD trips does not prove your earth connection.

A 30mA RCD should operate with a Zs up to 1667 ohms, if the Zs is anywhere near this then there is clearly a problem.

 

[Aaron b]

Depending on the design, RCDs are usually provided as additional protection not for fault protection. If you proved your earth was good (Ze by measuring or enquiry) and measured R1+R2, Zs can be calculated (some special individuals think it's to dangerous to carry out live tests).

I assume you were referring to ramp or trip time tests. They induce a current into the CPC which could create a voltage above 50V if the earth isn't good enough, modern meters have to stop the test if this occurs but it could potentially be dangerous. This is why R2 is proven before live testing.

I'm not sure if it is relevant but the test button doesn't short to earth it connects the outgoing line to the upstream neutral to create an imbalance.

 

[pc1966]

One point already made is if the ELFI is unusually high it typically means something is wrong. Something that ought to be fixed and not left to the added complexity of the RCD to protect folks against.

More generally though for lower current final circuits if you can't meet the Zs for disconnection then often you are going to be failing on VD as well. If you assume R2 and R1 are roughly the same, and much larger than the supply ELFI, then if you meet a 5% VD then your fault current is going to be around 20 times the working current, enough to (just) meet disconnection on even a D-curve MCB.

Of course if your R2 is higher, say T&E with the reduced size CPC, then that is no longer true but for typical domestic circuits on B-curve MCBs it will be easily met. In fact the On-Site Guide books Table 7.1(ii) makes it simple to check.

There are good arguments for high current final circuits or sub-main where you struggle to meet the ELFI due to the supply Ze value, even with easier OCPD such as BS88 fuses, where an RCD is quite a reasonable design solution.

 

[pc1966]

America and Canada don't test EFLI or the Ohms to Ground but they do check for continuity of earth

But they also had lots of fires so went down the AFDD / AFCI route years ago.

Why do good design when you can spend more money on fancy devices to mitigate that?

 

[pc1966]

I think the UK is unusually strict in how Zs is designed and tested, for example Aus/Nz use "typical" values for the MCB trip point and voltage, where as we use the worst-case combinations. It might be historically down to actually meaning to meet worst-case disconnection times on the OCPD. After all we were commonly TN supplies where that is a practical and reliable approach, where as EU was often TT so had to use RCDs (or VOELCB in dark days of past) to achieve disconnection.

 

[pc1966]

If you want the TL;DR version then "it is safer".

No single point of failure as you are able to disconnect on both the reliable thermal-magnetic MCB (or equivalent of an RCBO) and on the earth leakage detection electronics in the RCD (or equivalent part of an RCBO).