Zs values?

[IQ Electrical]

A common source of misunderstanding is that of either specifying or measuring values of ELI where the circuit also has an RCD fitted. ELI measurement under these circumstances is a futile excercise.

The circuit will have been checked for continuity, and this is all that is needed together with, of course, functional checks of the RCD.
This criterion satisfies requirements for automatic disconnection.

The subject is somewhat confused by the inclusion of RCBO's in table 41.3 and for clarification, circuits with RCBO's do not need to meet the specified ELI values.

 

[andyb]

If the maximum Zs value for a circuit in a TN system cannot be met, the circuit may be protected by a 30ma RCD. 531.3.1

I'm not sure that you can use that in a domestic situation Sintra, 531.3.1 does not deal with circuits but equipment in certain parts of a circuit.
So if you have a high Zs on a final ring circuit for example, you cannot just use the rcd as your main protection in my opinion.
Though I am of course open to being shown otherwise.

 

[Sintra]

Personally i don't see any reason not to as it has been done like this on a TT for years.

 

[nickblake]

I'm not sure that you can use that in a domestic situation Sintra, 531.3.1 does not deal with circuits but equipment in certain parts of a circuit.
So if you have a high Zs on a final ring circuit for example, you cannot just use the rcd as your main protection in my opinion.
Though I am of course open to being shown otherwise.

well if you check on how to correct a High Zs reading there are 2 main options increase the size of your cpc or install and RCD , so in theory if you have an installation thats protected by an RCD you can have higher than the permitted Zs readings , i must admit i prefer to get the readings withing the permitted levels

 

[Guest123]

i must admit i prefer to get the readings withing the permitted levels

Absolutely.

Unless it's a TT system there is no reason at all not to have Zs well within limits. A good design leads to a compliant install without the need for an RCD to cover a bad one.

The amount of times I've heard "dont worry about that there's an RCD on it anyway"

 

[IQ Electrical]

So if you have a high Zs on a final ring circuit for example, you cannot just use the rcd as your main protection in my opinion.

You are correct regarding using the RCD as your main protection but with an RCBO, your overcurrent protection will be carried out by the 'O' part of the RCBO thereby satisfying the requirements for automatic disconnection.

 

[Sintra]

Absolutely.

Unless it's a TT system there is no reason at all not to have Zs well within limits. A good design leads to a compliant install without the need for an RCD to cover a bad one.

The amount of times I've heard "dont worry about that there's an RCD on it anyway"

Agree with this also. On a new circuit it should never happen if designed correctly but it can be a quick fix on an existing circuit with a high Zs.

 

[jeremy]

I'm getting confused (though let's face it , it doesn't take much). The table in the osg gives values for 60898 and 61009 which are mcb and rcbo. rcds without overcurrent protection are covered by 61008 so aren't included in this table I presume. If the table says it's the max value isn't that what I should be putting down?

 

[SimpleSimon]

I'm not sure that you can use that in a domestic situation Sintra, 531.3.1 does not deal with circuits but equipment in certain parts of a circuit.
So if you have a high Zs on a final ring circuit for example, you cannot just use the rcd as your main protection in my opinion.
Though I am of course open to being shown otherwise.

You are right, the RCD is not to be used as main protection. But when installing an RCD as regards to High Zs values it's classed as additional protection which is allowed.
Your main protection is still by ADS.
Personally I agree with Sinatra. A new installed circuit should nit be reading a high Zs. But, if I was coming across high readings on a PIR that where protected by an RCD I'd probably record the 1667 value.

 

[nickblake]

Agree with this also. On a new circuit it should never happen if designed correctly but it can be a quick fix on an existing circuit with a high Zs.

Me to i recently did a rewire and due to restricted access cable runs were longer than id have liked and the calculated readings of 2 ring mains were a little higher than id have liked , so the rings were wired in 4mm twe instead of 2.5 , kept the readings low , it was a relativly small house but made of stone and a funny shape

oh and jeremy dont worry im always confused your not the only one

 

[nick611]

yes on tt system if ze less than 1667 then everythink ok.because rcd will disconnect in 0.2 thus satisfying 0.4 disconnection time

 

[Jimmy Boy]

415.1.2

The use of RCD's is NOT recognised as a sole means of protection and does not obviate the need to apply one of the protective measures specified in sections 411 to 414.

To my mind that means getting your Zs and disconnection times within those specified irrespective of RCD's (TT's being the exception)

 

[IQ Electrical]

yes on tt system if ze less than 1667 then everythink ok.because rcd will disconnect in 0.2 thus satisfying 0.4 disconnection time

Did you read through this before you posted?

 

[nick611]

the maximum zs reading on a tn circuit incorporating an rcd shall be 1667 as per iee regs.therefore zs readings redundant on 99% of domestic installations

 

[Jimmy Boy]

the maximum zs reading on a tn circuit incorporating an rcd shall be 1667 as per iee regs.therefore zs readings redundant on 99% of domestic installations

Reg number please so I can read up