Maximum Zs values for BS3871

gazdkw82 · Jan 23, 2019

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

The Ghost · Jan 23, 2019

This is from the Unite guide, a great little pocket book. Much the same as OSG except it is £7.

[ElectriciansForums.net] Maximum Zs values for BS3871

Deleted member 26818 · Jan 23, 2019

I believe the NICEIC/Electrical Safety Council published a chart showing the values back in 2008.
Here’s a link to the 2011 version:
https://www.----------------------------/media/1627/master_earthfaultlooptable.pdf

gazdkw82 · Jan 23, 2019

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

Deleted member 26818 · Jan 23, 2019

The one I posted a link to, shows the 80% values.

Taylortwocities · Jan 23, 2019

gazdkw82 said:
Those 2 references contradict each other? The unite guide says it's 0.719 and the other chart says 0.58??

My guess is that the NICEIC figures include the 80% rule of thumb malarky.

0.719 x 80% = 0.57

Taylortwocities · Jan 23, 2019

spinlondon said:
The one I posted a link to, shows the 80% values.

Snap!

gazdkw82 · Jan 23, 2019

Ah... Lovely.

Thankyou

Strima · Jan 23, 2019

Don't forget to adjust for Cmin... (0.95)

The Ghost · Jan 23, 2019

And bear in mind they are adjusted values for 230v instead of 240v.

gazdkw82 · Jan 23, 2019

It's just for a minor works cert so no need for Cmin

Strima · Jan 23, 2019

gazdkw82 said:
It's just for a minor works cert so no need for Cmin

Why not?

westward10 · Jan 23, 2019

Type 3 was on the same Table as a type C.

gazdkw82 · Jan 23, 2019

Strima said:
Why not?

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

westward10 · Jan 23, 2019

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 · Jan 23, 2019

gazdkw82 said:
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 · Jan 23, 2019

Strima said:
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 · Jan 23, 2019

gazdkw82 said:
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 · Jan 23, 2019

Strima said:
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 · Jan 23, 2019

gazdkw82 said:
Ah... Sorry. My mistake.

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

Correct. Do you know why Cmin was brought in?

gazdkw82 · Jan 23, 2019

Differences in voltage supplies?

Strima · Jan 23, 2019

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 · Jan 23, 2019

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

gazdkw82 · Jan 24, 2019

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.