Maximum Zs values for BS3871

[gazdkw82]

Where can I find the max Z's value for a BS3871, type 3, 32A MCB?

Iv had a look through BS7671 and done some searching online but I cannot find it. IV found some tables with listed Zs values for a 3871 but I have no idea where they originate from or how old

 

[Strima]

It's not required. It asks for the max Zs values listed in BS7671.

But the tables for BS3871 are not listed within BS7671 which already have Cmin applied as of the 17th Amdt 3.

So to ensure the circuit complies with current regulations you would have to apply Cmin and then 80% to take into account thermal compensation.

How is Cmin applicable to any works you carry out on an existing circuit?

 

[gazdkw82]

But the tables for BS3871 are not listed within BS7671 which already have Cmin applied as of the 17th Amdt 3.

So to ensure the circuit complies with current regulations you would have to apply Cmin and then 80% to take into account thermal compensation.

How is Cmin applicable to any works you carry out on an existing circuit?

Bs7671 does not have the rule of thumb applied. Onsite guide does. The minor works is asking for values from BS7671, therefore it's asking for values without the rule of thumb applied.

 

[Strima]

Bs7671 does not have the rule of thumb applied. Onsite guide does. The minor works is asking for values from BS7671, therefore it's asking for values without the rule of thumb applied.

I'm not talking about the rule of thumb, I'm talking about Cmin.

 

[gazdkw82]

I'm not talking about the rule of thumb, I'm talking about Cmin.

Ah... Sorry. My mistake.

So it's 0.95 correction factor (5%)?

 

[Strima]

Ah... Sorry. My mistake.

So it's 0.95 correction factor (5%)?

Correct. Do you know why Cmin was brought in?

 

[gazdkw82]

Differences in voltage supplies?

 

[Strima]

With most faults on an installation, the current is proportional to the supply voltage.

The intention of using Cmin when calculating the allowed loop impedance, is to ensure that sufficient fault current is available for the protective devices to operate within the required disconnection times at the minimum voltage the DNO is allowed to deliver - currently 6% below nominal.

 

[Andy78]

Table B6 in the OSG gives max Zs values for BS3871, BSEN60898, and BSEN61009.

 

[gazdkw82]

I spoke with guardian today. They informed me that R1+R2 must be taken from the point of circuit origin.

I have emailed all higher management/technicians and informed them that this is the standard requirement for testing.

 

[Ian1981]