why does this circuit not meet the correct Zs value ?
poor design ? undersized earthing ? too long a cable run ?
i'm already frothing at the mouth in fury.
;-)

Could be any or all of the above.. :) But purely hypothetically speaking, doesn't providing an RCD on the circuit make the max Zs requirements redundant?
 
I wouldn't design a circuit (TN-S) and relay solely on the rcd to meet disconnection times, ie wouldn't use the 1664 ohm Zs figure. Unless TT. Now if I was testing and the measured Zs was just over the max allowed and the circuit was rcd'd then I may just let it go, as long as understood why it was a bit to high
 
NO, it doesn't make ADS requirements redundant!! RCD's are classified as Additional protection, NOT the ONLY protection!!!

Of course not the only protection, but if they operate faster than a fuse/mcb in an earth fault situation , can they be used as a way to meet disconnection times when the primary protective device is compromised by a high Zs?

In other words, can they be used as a way to avoid running an additional cpc to a circuit if the existing R1+R2 doesn't meet the max Zs on its own?

Is there a specific Reg that forbids this?
 
Could be any or all of the above.. :) But purely hypothetically speaking, doesn't providing an RCD on the circuit make the max Zs requirements redundant?
well (and i hate to say it).....but yes it does......
look, i hate the idea of just grabbing the `cure all` RCD of the shelf to cover crap design...i`m sorry, but i`v got to say it....
 
In answer to the original question,yes it is permitted to use an RCD for earth fault protection....411.4.9 is the one you want.
Of course if an excessively high Zs is recorded on a TN system something is amiss which should have been picked up long before Zs is even measured because Zs should be equivalent to R1+R2 + Ze......which should already have been ascertained.

Where an RCD can be useful for earth fault protection on a TN system is where a circuit designed for a type B(for example)...needs to be changed to a typeC or D due to transformers or other high inrush equipment installed.
 
The problem with RCD's is that they are extremely unreliable. I bet every sparky on here has come across non functioning RCD's. They are great when they work but I certainly wouldn't have it as the only means of protection.
 
If you take this argument to its logical conclusion then we should be able to disregard all max Zs calculations in 7671 and assume that all circuits have a max Zs of 1667 Ohms if protected by an RCD

Max Zs is there to ensure that the resistance to earth is low enough to allow the circuit breaker to operate and disconnect the circuit accordingly (and high enough so it doesnt knacker the MCB) As stated above an RCD is there as additional protection. Dont mix up disconnection times (MCB) and tripping times(RCD), they are different
 
Certainly within a domestic scenario where virtually every circuit ends up coming under RCD protected as either a direct result of, or a by-product of, additional protection requirements, the need to measure R1+R2 to show disconnection times are met is somewhat redundant. Not saying it’s right for a variety of reasons that have been previously stated but the regs do permit it.

The RCD is measuring a difference between L/N and provided you are getting a flow to earth somewhere it will trip once the magical rating has been reached (e.g. 30mA)—you will of course have proved you have an earth path/CPC as part of your continuity checks and the later RCD check will prove the RCD trips as required.

Clearly R1+R2 has a usefulness for proving continuity of conductors and meeting R1&R2 figures would also be reassuring as a back-up to the failing of an RCD. Plus of course all the forms out there seem to have it as a measurement required to be recorded anyway!!

Me, I still prefer to make sure my circuits meet R1+R2 regardless of RCD protection .......
 
il say different , doesnt have to flow to earth can flow onto an non earthed material just has to flow somewhere anywhere to unbalance in and out and wallla rcd trips.

as this is an old install and we know you didnt design this circuit with your head up your twooot , you can use rcd to cure, better than nothing and rewiring circuit if is all concealed.
 
The Electricians Forum Guide to Circuit Design
==================================


Step 1
=====

Fit rcd.






there is no step 2.


I'd be grateful if admin could make this a sticky , saves all that faffing about with regs books and equations.



;-)
 
If you take this argument to its logical conclusion then we should be able to disregard all max Zs calculations in 7671 and assume that all circuits have a max Zs of 1667 Ohms if protected by an RCD

Max Zs is there to ensure that the resistance to earth is low enough to allow the circuit breaker to operate and disconnect the circuit accordingly (and high enough so it doesnt knacker the MCB) As stated above an RCD is there as additional protection. Dont mix up disconnection times (MCB) and tripping times(RCD), they are different

You are missing the point though....Zs is only measured after R1+R2 has been measured on a new circuit....if it's a TN system,and continuity and Ze readings are within expected limits,then Zs will be satisfactory. If it's not something is amiss which has nothing to do with earth loop impedance and should have been rectified long before the ELI tester comes out.
Another point often forgotten by the 'you cant use an RCD for earth fault protection,it's additional protection only' brigade is that nearly all TT systems in the UK rely solely on one or more RCD's for earth fault protection.
 

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Green 2 Go Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses Heating 2 Go
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

Advert

Daily, weekly or monthly email

Thread Information

Title
When Zs is too high...
Prefix
N/A
Forum
UK Electrical Forum
Start date
Last reply date
Replies
40
Unsolved
--

Advert

Thread statistics

Created
Crosswire,
Last reply from
growler,
Replies
40
Views
25,082

Advert

Back
Top