Is there a max Zs for lighting circuits?

[mak]

I can't find anything related to this in the regs so I probably misunderstood the question. The way I see it the max Zs for any lighting circuit depends on the circuit breaker for that circuit.
Say we have a lighting circuit with 100x36W ceiling lights which gives us a load of 16A, we choose the breaker of 16A type C, so the max Zs for that circuit will be 1.09ohms. Or with the ambient temperature correction factor 1.09 x 1.06 = 1.15

 

[davesparks]

The max Zs is governed by the protective device, it doesn't change for lighting circuits.

Where did you get 1.06 from? Temperature correction for testing carried out at ambient temperature of 20C instead of max operating temperature of 70C is achieved by multiplying by 0.8

 

[pc1966]

3.6kW is a lot of lights, if they are simultaneously switched you are going to have trouble with a 16A C-curve breaker anyway!

But ignoring that for just now, the OSG Table B6 has 1.09 ohm for that type, that already has a 0.8 factor included for the typical 70C max operating temperature.

The big blue book (Appendix 3, Figure 3A5) has the trip point as 160A to get 0.4s (in reality, anything below 5s is reached by the magnetic trip and is closer to 20ms or less). So the Zs would be computed from:

Zs = Umin / Itrip = 0.95 * 230 / 160 = 1.37 ohm

Then allowing for hot cables during operation 0.8 * 1.37 = 1.09 ohm which would be your measurement when cold to verify the system. Or design using cold resistance values and assuming your cable is not hotter than 70C.
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Also the Zs values without hot correction are given in the BBB in Table 41.3 for MCB/RCBO

 

[mak]

The max Zs is governed by the protective device, it doesn't change for lighting circuits.

Where did you get 1.06 from? Temperature correction for testing carried out at ambient temperature of 20C instead of max operating temperature of 70C is achieved by multiplying by 0.8

I used 25C. Table B8 onsite guide. Where did you get the 0.8?

 

[Baddegg]

I used 25C. Table B8 onsite guide. Where did you get the 0.8?

0.8 is the 80% correction

 

[mak]

3.6kW is a lot of lights, if they are simultaneously switched you are going to have trouble with a 16A C-curve breaker anyway!

But ignoring that for just now, the OSG Table B6 has 1.09 ohm for that type, that already has a 0.8 factor included for the typical 70C max operating temperature.
Also the Zs values without hot correction are given in the BBB in Table 41.3 for MCB/RCBO

I get it. BBB has no correction factor applied unlike the on-site guide. So table B8 factor is optional?

The big blue book (Appendix 3, Figure 3A5) has the trip point as 160A to get 0.4s (in reality, anything below 5s is reached by the magnetic trip and is closer to 20ms or less). So the Zs would be computed from:

Zs = Umin / Itrip = 0.95 * 230 / 160 = 1.37 ohm

Then allowing for hot cables during operation 0.8 * 1.37 = 1.09 ohm which would be your measurement when cold to verify the system. Or design using cold resistance values and assuming your cable is not hotter than 70C.

That makes a lot of sense. Thanks.

 

[davesparks]

I get it. BBB has no correction factor applied unlike the on-site guide. So table B8 factor is optional?

That makes a lot of sense. Thanks.

The factor isn't optional, it needs to be applied when required.

The values in BS7671 are for conductors operating at their rated operating temperature, usually 70C.
Testing is carried out, normally, with the conductors at ambient temperature (around 20C).
As the resistance of the conductors increases with temperature you need to adjust the values for operating temperature to find out what the equivalent value at ambient temperature is.
0.8 is the factor used for this.

There are a great many flaws with this method, but to be more accurate would require every single circuit to have its maximum Zs calculated seperately for its own unique circumstances which would be impractical in the vats majority of cases.
You can work it out accurately for any circuit if you feel it is necessary/justified, but unless it's a large circuit with tight tolerances and big financial implications it is unlikely to be worth the effort.

 

[BruceBanner]

Max zs applies to any circuit, regardless of load, its all in your osg or bs7671. Not trying to berate or put you down in any way, but am curious as to why you thought it may not apply to lighting. Any questions please fire, away, it's good you are asking in the first place.