Zs values?

[IQ Electrical]

That's exactly right, many people incorrectly assume this is just for TT systems.

 

[telectrix]

so, one advantage of the 17th is, with everything now on RCD ( well almost), we don't have to remember all those max. Zsvalues for different MCBs. every cloud has a silver spoon!

 

[malcolmsanford]

RCD/RCBO circuits will have the Zs part of the form filled in as 1667 as that is the "Zs" as such for the circuit.

On domestic the chances are that as the board that is fitted will be a dual board then you also have overload protection by MCB and that circuit will have your Zs, and you will enter the Zs results. A lot of argument is now happening that even on dual boards you can fill the Zs for each circuit as 1667 as all circuits are RCD protected. IMO in a few years once we are all use to this we will all fill the schedule of resutls with RCD protection as 1667. I have to admit until recently I use to fill the Zs part in for each circuit as measured but I will start doing it the 1667 method.

Obviously on PIR's for older installtions, and installtions not needing RCD protection you will still do Zs as normal.

 

[telectrix]

even so, still put in the measured Zs in it's column, but max. as 1667.

 

[malcolmsanford]

even so, still put in the measured Zs in it's column, but max. as 1667.

Yes 1667 everything that as RCD protection

 

[malcolmsanford]

That's exactly right, many people incorrectly assume this is just for TT systems.

I was told recently by a NIC man that TT is 1667 and TN is 1.6 Kohm !!! After I stopped laughing at him he told me that as he was an assessor that was what should be written

 

[IQ Electrical]

Well that's your answer Malcolm, the Lord hath spoken!

And on the seventh day, the NICEIC wrote the regulations!

 

[andyb]

1667 is for TT systems.
On a TNS system the mcb or rcbo is your method of complying with 411.3.2.2 & you use table 41.3
The rcd is for additional protection so 1667 does not come into it.

Table 41.5 is for TT systems.

 

[jam2020]

errrr whats the diffinitive answer here chaps??

 

[Sintra]

1667 is for TT systems.
On a TNS system the mcb or rcbo is your method of complying with 411.3.2.2 & you use table 41.3
The rcd is for additional protection so 1667 does not come into it.

Table 41.5 is for TT systems.

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

 

[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