# If you need a Zs figure for a type B,C or D ......

Discussion in 'Electrics UK' started by festa, 2 Apr 2007.

1. ### festa

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If your ever caught short without your `BIG BROWN BIBLE BS7671`and need to know a Z`s figure use this simple formula.....

For a type B MCB....
1.Multiply the nominal setting of MCB by 5
Then using Ohms law...
2.Divide 240 (volts) by the sum you got in the above calculation.

I.E for a 25 amp type B

25A x 5 = 125A
240V / 125A = 1.92 ohms

For a type C MCB substitute the 5 for a 10

25A x 10 = 250A
240V / 250A = 0.96ohms

For a type D MCB substitute the 10 for a 20

25A x 20 =500A
240V / 500A = 0.48ohms.

3. ### Jim2287

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Or you can do following, as is shown in table 41B2, BS7671 16th edition

Type B MCBs BSEN 60898 and RCBOs BSEN 61009

48 / Current rating of MCB

Type C MCBs BSEN 60898 and RCBOs BSEN 61009

24 / Current rating of MCB

Type D MCBs BSEN 60898 and RCBOs BSEN 61009

12 / Current rating of MCB

4. ### Spark123

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Don't forget to also compensate for conductor temperature when comparing with measured results, when using copper conductors for a temperature drop of 60ºC i.e. from 70ºC to 10ºC the max Zs should be divided by 1+(60x0.004)=1.24 or multiplied by 0.8 (figures in OSG).

5. ### Jim2287

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NIC told us that you dont need to do that, as the ones in regs are closer to real maximums ... dunno if thats right or not

thats probably just because we do domestic and temps dont change that drastically

6. ### Spark123

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Isn't there a chart with maximum values for measured Zs printed on some of the NICEIC pads?

8. ### RF Lighting

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Yep, but I don't have one at home

9. ### DESL

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But they don't write the Regs!!!!!!!

10. ### Spark123

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I'd think it a bit odd that they go to the trouble of printing the figures on their pads only to say don't use them

11. ### RobFurn

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The OSG quotes figures that have been corrected i.e
BS7671 (brown book) table 41B2 quotes 1.5ohms for 32A B type MCB but table 2D of the OSG (page 92) quotes 1.2ohms for a B type MCB

(1.5 x0.8 = 1.2)

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