Discuss Maximum Zs values for BS3871 in the Electrical Courses and Electrical NVQ's area at ElectriciansForums.net

gazdkw82

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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
 
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gazdkw82

gazdkw82

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Those 2 references contradict each other? The unite guide says it's 0.719 and the other chart says 0.58??
 

Strima

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Don't forget to adjust for Cmin... (0.95)
 
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And bear in mind they are adjusted values for 230v instead of 240v.
 

westward10

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You would need to look in a 16th Ed to find them as I said they were the same Table as a type C.
 

Strima

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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?
 
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gazdkw82

gazdkw82

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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

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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.
 

Strima

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Strima

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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.
 
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gazdkw82

gazdkw82

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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.
 

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