Max Zs TN system can’t achieve

[Bradley6969]

Sorry further on from last post, TNS system, Zs can’t be achieved on final circuits, getting 1.08 Zs on a B63 0.69 but protected by 167 ohms, poor design cable runs are too long and R1+R2 proves this, now I’ve spoke with my boss about what to record in Max Zs is it 0.69 or 167 ohms as all circuits are protected by 300mA RCD, also will MCB trip on overload having not met disconnection times? Sorry this one is confusing me

 

[buzzlightyear]

Bradley if you are paid by your boss then he should tell you what to put down on the test sheets .
how ever you record the high test value and on the that test sheet well above max impedance value .

 

[Bradley6969]

Bradley if you are paid by your boss then he should tell you what to put down on the test sheets .
how ever you record the high test value and on the that test sheet well above max impedance value .

So you’d record 0.69? For a B63

 

[Ian1981]

The overload part of a circuit breaker has nothing to do with the earth fault loop impedance, so yes you should still have adequate overload protection, 63 x 1.45 = 91.35 amps
This will be achieved if Ib In Iz is satisfied

 

[Bradley6969]

Obviously my 0.69 for a B63 I’m not going to achieve it so in Maximum Zs permitted by BS7671 I’m not sure wether to put 0.69 for the B63 or all the circuits are protected by 300mA RCD which covers 167 ohms, I suppose either doesn’t matter? Just want to know so I’m right before asking him
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The overload part of a circuit breaker has nothing to do with the earth fault loop impedance, so yes you should still have adequate overload protection, 63 x 1.45 = 91.35 amps
This will be achieved if Ib In Iz is satisfied

Thanks where’s 1.45 from?

 

[Ian1981]

Obviously my 0.69 for a B63 I’m not going to achieve it so in Maximum Zs permitted by BS7671 I’m not sure wether to put 0.69 for the B63 or all the circuits are protected by 300mA RCD which covers 167 ohms, I suppose either doesn’t matter? Just want to know so I’m right before asking him
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Thanks where’s 1.45 from?

Check out section 433.1 of bs7671 co-ordination between conductor and overload device.
Typically a circuit breaker has a ‘fusing factor’ of 1.45 that is it should disconnect on overload at 1.45 it’s rating within an hour.
Nothing to do with it’s maximum‘Zs’ of the device itself

 

[Bradley6969]

Check out section 433.1 of bs7671 co-ordination between conductor and overload device.
Typically a circuit breaker has a ‘fusing factor’ of 1.45 that is it should disconnect on overload at 1.45 it’s rating within an hour.
Nothing to do with it’s maximum‘Zs’ of the device itself

Top man thanks I’ll have a look

 

[telectrix]

. a B63 will operate in approx 50 minutes @91A, thus complying with the 1.45In in 1 hour. . however, the overload current will be reduced somewhat by the increased resistance of the L and N conductors... might just be enough to make it non-compliant.

 

[Ian1981]

. a B63 will operate in approx 50 minutes @91A, thus complying with the 1.45In in 1 hour. . however, the overload current will be reduced somewhat by the increased Zs.. might just be enough to make it non-compliant.

How will the overload current be reduced by an increased Zs?
Overload current is just that , an overload by drawing more current than the ocpd or the conductors during non fault conditions.
Zs relates to an Earth fault between the line and cpc/ exposed conductive parts

 

[telectrix]

How will the overload current be reduced by an increased Zs?
Overload current is just that , an overload
Zs relates to an Earth fault between the line and cpc/ exposed conductive parts

i put that badly. meant to say that both the L and N conductors will have a higher than expected resistance, so that in the case of an overload, the conductors will reduce it's magnitude by a small amount. just edited previous post.

 

[cliffed]

No problem with earth protection,but overload/short circuit may be compromised,due to short circuit current,& disconnection times

 

[Ian1981]

If you measure the line to neutral loop then unless the conductors are the same size and length as the cpc you’ll probably find the line to neutral loop has a lower impedance

 

[davesparks]

i put that badly. meant to say that both the L and N conductors will have a higher than expected resistance, so that in the case of an overload, the conductors will reduce it's magnitude by a small amount. just edited previous post.

That doesn't matter though, the requirement is only that if an overload of 1.45 times occurs it disconnects in an hour. We don't have to do anything to try to ensure that an overload achieves a particular value or disconnection time.

 

[Bradley6969]

Obviously my 0.69 for a B63 I’m not going to achieve it so in Maximum Zs permitted by BS7671 I’m not sure wether to put 0.69 for the B63 or all the circuits are protected by 300mA RCD which covers 167 ohms, I suppose either doesn’t matter? Just want to know so I’m right before asking him
Anyone???

 

[cliffed]

Agree,the Rcd will protect earth fault ,the mcb may not protect the cabling on short circuit conditions.
Due to short circuit fault conditions,with the loop impedance not complying.

 

[Bradley6969]

Spoke to my boss, he’s said to put 167 ohms for RCD and all MCBs Personally I’d put 0.69 ohms and note on EICR?
What would you guys do? I’d rather know for next time, if it was TT and had 300mA RCD I’d put 167 for all MCBs but on TNS I’d put the Zs for breaker?
Thanks

 

[cliffed]

Defo 0.69,what about changing the mcb,to accommodate Zs .

 

[Bradley6969]

Defo 0.69,what about changing the mcb,to accommodate Zs .

He said only put 0.69 if you can achieve it on the circuit if not rely on the Zs for the RCD which I don’t agree with
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Defo 0.69,what about changing the mcb,to accommodate Zs .

Circuits are too long and feed 4/5 caravans with 16amp sockets so can’t change MCB just been poorly designed I just want to know what every one else would put rather be right as my name will be on it

 

[buzzlightyear]

The test is maxt out. how can you put down 0.69 on the test sheet .

 

[Ian1981]

I agree with your boss as the mcb is not giving you earth fault protection for the circuit so the rcd used will determine the maximum zs that will give you your required disconnection time

 

[Bradley6969]

Sorry I just wanted opions, to be honest Max Zs on test sheet and having 1.01 for example doesn’t look right, so I’ll go with 167 ohms
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I just wanted to know so next time I get it right