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# Domestic45A breaker max Zs values

Discuss 45A breaker max Zs values in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

#### paj

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Hi all,
Why is there no max Zs value for a 45A type B breaker in table 41.3 of 18th edition? What value should be used as max value on schedule of test results?
Cheers.

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B

#### Bobster

Because it can't possibly list all the different sizes available, just the most common.

You can either calculate it, or look at manufacturer data.

#### Ian1981

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0.97 ohms using 218.5/225 for a type B mcb

#### Sintra

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I got 0.93 using 225/230x95%

#### Ian1981

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V/I = 0.97
230 x 0.95=218.5
45 x 5=225
218.5/225=0.97

Are you not supposed to multiply in the equation first?

O

#### Octopus

My certificate software says 0.97

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#### Marcus Vaughan

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Yes you do have to multiply the 5 by the 45 first.

#### ELECNEWT

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In addition to C min (0.95) to take account of voltage variation, do you have to incorporate a factor of 0.8 for temperature effects?

I.e. 230*0.95/ 5*45=218.5/225 =0.97

But to account for temperature effects the corrected value of Zs is 0.97*0.8 =0.77 ohms.

#### Marcus Vaughan

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In addition to C min (0.95) to take account of voltage variation, do you have to incorporate a factor of 0.8 for temperature effects?

I.e. 230*0.95/ 5*45=218.5/225 =0.97

But to account for temperature effects the corrected value of Zs is 0.97*0.8 =0.77 ohms.
Yes. As a rule of thumb. More accurate compensations for temperature differences can be seen in Guidance Note 3, though this is rarely needed unless your design is getting close to limits.

The OP asked why 45A wasn't shown in the BBB, and these are the uncorrected values that everyone has supplied.

On certs I tend to state the corrected value for max Zs and put a footnote to say what I've done. To me this makes sense because further along you have your actual measured Zs and hence you get a quick cross check that your circuit complies. The question of whether to put corrected or uncorrected Maz Zs on Schedules of Inspections has been asked before and created some debate.....

#### ELECNEWT

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On certs I tend to state the corrected value for max Zs and put a footnote to say what I've done. To me this makes sense because further along you have your actual measured Zs and hence you get a quick cross check that your circuit complies. The question of whether to put corrected or uncorrected Maz Zs on Schedules of Inspections has been asked before and created some debate.....[/QUOTE]

Thank you Marcus. It does make sense.
As a supplementary question:when calculating voltage drop do you always use the factor of 1.2 to account for temperature effects?

#### paj

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Thank you all for your prompt and useful replies.
I accept the point made by Rob about they can't include every device but I was surprised one as common as 45A wasn't there.
I understand that Cmin was introduced to allow a "safety factor" to the BS 7671 Zsmax values in consideration of possible supply voltage fluctuation i.e. the minimum voltage available may be as low as 95% of nominal (218.5V) under certain conditions.
We then use this min possible voltage divided by the operating current of the device, in this case 225A, to find the Zsmax which will ensure the device operates in the required disconnection time, even if the supply voltage is at its minimum. As per Marcus and Ian.
Moving onto the second bit of my query. This would be the "BS 7671 Zsmax value" to write in column 8 of the Schedule of Test Results if we follow the apparent meaning/spirit of the Regs? This value would still be subject to the 0.8 rule of thumb when comparing with measured test values. The footnote on the bottom of the schedule allows for values from other sources to be used if this is stated in column 25. An example could be taking an already corrected value from the OSG which still has the 45A Zs value listed.
I am not trying to be pedantic here but I am trying get to a correct firm procedure on how to deal with such issues. What is acceptable or not. I would agree totally with the logic of Marcus and Elecnewt, but it doesn't seem to fit with the intentions of the Regs. I am also thinking about lessening the confusion for those training in inspection and testing.

Thanks to all.

#### Ian1981

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Thank you all for your prompt and useful replies.
I accept the point made by Rob about they can't include every device but I was surprised one as common as 45A wasn't there.
I understand that Cmin was introduced to allow a "safety factor" to the BS 7671 Zsmax values in consideration of possible supply voltage fluctuation i.e. the minimum voltage available may be as low as 95% of nominal (218.5V) under certain conditions.
We then use this min possible voltage divided by the operating current of the device, in this case 225A, to find the Zsmax which will ensure the device operates in the required disconnection time, even if the supply voltage is at its minimum. As per Marcus and Ian.
Moving onto the second bit of my query. This would be the "BS 7671 Zsmax value" to write in column 8 of the Schedule of Test Results if we follow the apparent meaning/spirit of the Regs? This value would still be subject to the 0.8 rule of thumb when comparing with measured test values. The footnote on the bottom of the schedule allows for values from other sources to be used if this is stated in column 25. An example could be taking an already corrected value from the OSG which still has the 45A Zs value listed.
I am not trying to be pedantic here but I am trying get to a correct firm procedure on how to deal with such issues. What is acceptable or not. I would agree totally with the logic of Marcus and Elecnewt, but it doesn't seem to fit with the intentions of the Regs. I am also thinking about lessening the confusion for those training in inspection and testing.

Thanks to all.
Other sources of values can come from the actual manufacturer of the ocpd rather than using the generic values given for common devices listed in chapter 41 of BS7671

#### Wilko

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