Calculating Max Zs for BS 3871 MCB | on ElectriciansForums

Discuss Calculating Max Zs for BS 3871 MCB in the Australia area at ElectriciansForums.net

J

jud720

Hi there,

I am just trying to work out the Max Zs for a Type 1 13A
Page 58 OSG Table 7.5 for Type 1 do i multiply 13A by 4? :confused:
This gives me a Max Zs of 4.42Ohms :rolleyes:

Is this correct or am i way off

Appreciate any replies

Cheers

Justin
 
What is the protective device?

BS 3871 MCB's max Zs are on page 103, but they do not have 13A, they have 10 & 15A.

10A = Max MEASURED Zs of 4.64 ohms
15A = Max MEASURED Zs of 3.09 ohms


So you're numbers do look as expected..
 
Last edited:
Thank you for the reply

BS 3871 Type 1 13A

I was just wondering if i can apply the same calc as you do for 60898 as all is 0.1 disconnection

Page 103 are the OSG Zs not 7671 Max Zs

Regards

Justin
 
Yes, for BS 3871 type 1, multiply by 4.
For type 2 multiply by 7 and type 3 by 10 (Table 7.5 OSG).
Always use the worst case scenario.

Eg. for 15A BS 3871 type 1
15 x 4 = 60A
230 / 60 = 3.83ohms max Zs
Using rule of thumb to correct for temperature multiply by 0.8 to give 3.06 ohms

Good idea to memorise Table 7.5 OSG values, as is a easy way to get max Zs values if you dont have OSG or BRB to hand.

Hope this helps
 
Why is it on the elecsa Max Zs chart in the cert pads it says "N/A" for all BS3871 MCB's? not giving any Max Permited Zs..

This is also the exact same for the easy cert software, it auto puts N/A in the Max Zs box... surely you cant get away with N/A on the cert? so why doesnt it state the values like any other MCB?
 
Can't see how there getting away with that one mate, 3871 are far to common and it is disappointing that the BRB does seem to ignore this! Although there excuse will be manufacturers data, ÂŁ80 for a book is it ridiculous to expect these to be included?
 

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