Quiz Time :D

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3.The 20A type B would trip instantaneously at 5x20A=100A and the 16A type C would trip at 10x16=160A, so this shows that the 20A MCB would trip first.

Or to be a little more precise..

BS EN 60898 permits instantaneous tripping within the following ranges

TYPE B 3x In - 5x In

TYPE C 5x In - 10x In

TYPE D 10x In - 20x In

Re: Pg 50 Table 4.4, NICEIC Domestic Electrical Installation Guide, ISBN 0-9548791-1-2

& to meet max Zs for BS7671 the Upper PFC limits must not be exceeded to ensure instantaneous trip.

so as a quick ref table..

Amps........3 In......5 In.......10 In......20 In

_6............18.........30..........60.........120

10............30.........50.........100........200

16............48.........80.........160........320

20............60........100........200.........400

32............96........160........320.........640

45..........135.........225........450........900

50..........150.........250........500.......1000

20A Type B ... 60A .. 100A

16A Type C ... 80A .. 160A

So as Nicky quite rightly says..

In all reasonable probability to 20A Type B would trip first on a PFC type fault..

However there is the slight chance that the 16A Type C may sneak in during the 80A to 100A stage..

dependant upon manufactures tolerances!

Even if they were both Type B.. The 20A is still more likely to trip first!

The general rule of thumb for series connected circuit breakers to achieve the required discrimination

(or selectivity as some publication call it) is a 2:1 ratio of the current rating.

e.g.

a 10A & 20A of the same type would provide suitable discrimination.

a 20A & 45A of the same type would provide suitable discrimination.

whereas a 32A & 50A of the same type may NOT provide suitable discrimination.

Looks like I better go back over to "No-More-Nails" while DonkeyDong is still holding the door open! :_| :^O :x

 

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Oh dear...Looks like we're turning into Screwfix - SL I think you might fit in better over there

D.

Oh dear!

Time to play dumb and see how many different answers we get ...Its Friday night...let the fun begin! D.

And I thought you said..

Play dumb..

Different answers..

Let the fun begin.. ; \

I was sure I had met most of those criteria! X( :_|

Perhaps I was mistaken?? ?:|

 

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1) Do you record the RCD disconnection times for each circuit you've worked on (bearing in mind there is no RCD on DB2) ? (if required by the EIC)

What location description on the EIC sheet are you referring to?

Surely you would record the relevant figures for the relevant distribution board...

as per the "designation and location" of the CU / Dist' Board that you refer to on your schedule of test results...

Would you write details of the 20A and the 16A MCB on the same EIC results sheet?Obviously it is not good practice to have test results relating to two different boards on the same sheet..

This is where ye ole' addition sheets come useful?

If that Dist' board don't have a RCD... then the sheet relating to that board don't have no RCD details? eh. ?:|

I am not aware of any rule about the maximum number of addition sheets/records you can add i.e box "page 1 of [x]"

So long as you provide clarity and avoid confusion.

 

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2) The Zs is too high on one of your circuits in DB2 - Do you care as there is an RCD upstream ?

Before even getting onto that I would probably be concerned with compliance with 314.1 Division of Installation, into circuits to minimise hazards & inconvenience in the event of a fault.! :|

Then as long a suitable circuits are designed & installed worry about checking the Zs, and checking RCD trip rating x Zs <50v where appropriate.