TNS system Max Zs exceeded

[Bradley6969]

Local caravan site, 16/2C swa out have 3 or 4 Electric points with 2 x 16amp sockets on one circuit protected by B63 MCB now, TNS system Main RCD 100/300 mA protecting MCBs, now I’m testing and getting Max Zs of 0.99 when permitted is 0.69 so pointlet de rating to 50 amp, I don’t like the idea of relying on just one RCD as we know there unreliable but we’re covered for 1667 ohms, now I’m doing an EICR there’s nothing wrong with it, i just want to know if you’d put Max Zs for RCD so 1667 or for the protective device.
Poor design my best bet is if they use a split gland plate or split the earthing from sub circuits I’d TNS the supply and TT the outgoing but I haven’t checked the rest of the panels but I guess they will be the same but some are only a year old.
Thanks

 

[davesparks]

It is not 1667 ohms for a 300mA RCD on a TNS system, that figure is for a 30mA RCD on a TT system.

You can't base an EICR on your personal opinion about the reliability of an RCD. Relying on that RCD for fault protection does comply so you cant say much more about that.
However you may want to say something about the selectivity issue.

Why woukd you want to create a TT earth for these circuits? It will take a lot of work to get a better Zs than you have already got by doing this, you will almost certainly end up with a higher Zs and even greater reliance on an RCD which is a worse situation.

If its only a year old have you checked the EIC for this work to see if anything has changed or if your results match? One of the reasons we do an EICR is to be able to identify changes in the test results from previous tests to identify changes in the installation.

 

[buzzlightyear]

so pointlet de rating to 50 amp,

please explain to me that. nowing there is a b63 in

 

[Bradley6969]

It is not 1667 ohms for a 300mA RCD on a TNS system, that figure is for a 30mA RCD on a TT system.

You can't base an EICR on your personal opinion about the reliability of an RCD. Relying on that RCD for fault protection does comply so you cant say much more about that.
However you may want to say something about the selectivity issue.

Why woukd you want to create a TT earth for these circuits? It will take a lot of work to get a better Zs than you have already got by doing this, you will almost certainly end up with a higher Zs and even greater reliance on an RCD which is a worse situation.

If its only a year old have you checked the EIC for this work to see if anything has changed or if your results match? One of the reasons we do an EICR is to be able to identify changes in the test results from previous tests to identify changes in the installation.

Sorry mean 167 ohms and no old paper work for the site unfortunately

 

[Deleted member 105166]

Is the armour on 2-core SWA sufficient to be used as a CPC conductor? I recall something from John Ward recently, suggesting it may not be adequate due to the reduced diameter (specifically of 2-core SWA) and the 8x resistance of steel vs copper. On second thoughts, it may be he was saying it's inadequate to carry a submain where there's bonding?

 

[buzzlightyear]

on the test sheet put down above high impedance and investigation is in order .

 

[davesparks]

Sorry mean 167 ohms and no old paper work for the site unfortunately

That's still not right, it would be 167 on a TT system not on TNS.
The regulation on this changed with the 18th edition, you no longer use 50V in the calculation, you use 230V.

 

[Bradley6969]

That's still not right, it would be 167 on a TT system not on TNS.
The regulation on this changed with the 18th edition, you no longer use 50V in the calculation, you use 230V.

So put the MCB Max Zs would you note in comments that RCD is being used as Zs can’t be achieved?

 

[fairlight]

That's still not right, it would be 167 on a TT system not on TNS.
The regulation on this changed with the 18th edition, you no longer use 50V in the calculation, you use 230V.

Sorry but that is not correct.
411.4.5 states that an RCD may be used for fault protection.
411.4.204 states that where an RCD is used to satisfy the requirements of 411.3.2.3 the maximum values of Zs in table 41.5 may be applied.
411.3.2.3 specifically covers distribution circuits on a TN system.

 

[fairlight]

So put the MCB Max Zs would you note in comments that RCD is being used as Zs can’t be achieved?

The maximum permitted value of Zs is 167 ohms, that is the value you should apply. See the reg numbers I have posted above.

 

[Bradley6969]

The maximum permitted value of Zs is 167 ohms, that is the value you should apply. See the reg numbers I have posted above.

Thanks buddy I understand now, I’ve had a read through.
Thanks again

 

[Deleted member 105166]

Local caravan site, 16/2C swa out have 3 or 4 Electric points with 2 x 16amp sockets on one circuit protected by B63 MCB now, TNS system Main RCD 100/300 mA protecting MCBs

Just read this again - are you saying there are multiple 16A IEC60309 outlets, protected by a single 63A device? i.e. no downstream protection?

 

[Bradley6969]

Just read this again - are you saying there are multiple 16A IEC60309 outlets, protected by a single 63A device? i.e. no downstream protection?

Nope each socket has RCD and MCB protection, but most have 3 Electric points fed on one B63 now looking at the results 2 points comply with Max Zs so less than 0.69 but say the last one on each circuit it doesn’t meet disconnection time for the B63 which I’m guessing is due to the cable run and volt drop all glands are fine and the panel has a reading of 0.38 so poorly designed should be on smaller breakers and less points on a circuit

 

[buzzlightyear]

3 Electric points fed on one B63

a bad design should be three circuits 16amp each .

 

[GBDamo]

One of the reasons we do an EICR is to be able to identify changes in the test results from previous tests to identify changes in the installation.

One day i will ask the question " can i see you previous EICR/EIC" and the answer will be yes.

I will die of a heart attack, but happy.

 

[davesparks]

Sorry but that is not correct.
411.4.5 states that an RCD may be used for fault protection.
411.4.204 states that where an RCD is used to satisfy the requirements of 411.3.2.3 the maximum values of Zs in table 41.5 may be applied.
411.3.2.3 specifically covers distribution circuits on a TN system.

It is correct, and the regulations you have referred to even tell you this.

167 is the value for a 300mA RCD on a TT system, this is calculated based on a 50V touch voltage.

The 18th edition changed the requirements for an RCD on a TN system to now use 230V (x Cmin) in the calculation.
So the correct value for the OPs situation is 728.3
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The maximum permitted value of Zs is 167 ohms, that is the value you should apply. See the reg numbers I have posted above.

That is the value for a TT system, not for a TN system

 

[fairlight]

Where exactly does it say the requirements have changed for a TN system?
I repeat 411.4.204 clearly states table 41.5 shall be used where an RCD is used to satisfy the requirements of 411.3.2.3.
411.3.2.3 refers to distribution circuits on TN systems.
Table 41.5 give a maximum value of Zs of 167 ohms.
Please give the regulation numbers you are referring to and if I'm wrong I'll hold my hands up.
There is no need for dislikes either, a disagree and backing up your assertion with relevant reg numbers will do.
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If you are referring to 411.4.202 that concerns circuit breakers, under the definition of a circuit breaker a pure RCD will not break abnormal or short circuit currents so is not classed as a circuit breaker so that regulation does not apply to RCD's