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Topic Title: Type 1, 2, 3 MCBs and max Zs
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Created On: 01 September 2013 08:42 PM
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 01 September 2013 08:42 PM
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Smith249

Posts: 362
Joined: 09 September 2004

Good evening.

Could someone explain the older MCB types modern equivalent ?, also the max Zs for them.

Many thanks.
 01 September 2013 08:53 PM
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geoffsd

Posts: 392
Joined: 15 June 2010

Page 72 in OSG
 02 September 2013 10:12 AM
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AJJewsbury

Posts: 11768
Joined: 13 August 2003

Could someone explain the older MCB types modern equivalent ?

Mostly the difference is the magnetic trip rating -
B types - 3 to 5x In
C types - 5 to 10x In
D types - 10 to 20x In
where as the old types:
type 1 - 2.7 to 4x In
type 2 - 4 to 7x In
type 3 - 7 to 10x In

So you can calculate max Zs using Ohm's Law and the current needed to reliably achieve the upper rating. E.g. a 30A type 2 will guarantee to trip at 7x In = 7x30 = 210A, hence 1.1 Ohms (or 0.8x = 0.88 Ohms if you want it corrected to room temperature using the usual rule of thumb).

- Andy.
 02 September 2013 11:20 AM
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Avatar for Parsley.
Parsley

Posts: 1090
Joined: 04 November 2004

here's an example 3871 type 2 5amp MCB

Type 2 I.T.C (see table below) = 4 to 7 use 7.
Multiply by current rating.
Divide into Uo (230V)

7 x 5 = 35 A
230/35= 6.57 ohms (as per AJ use rule of thumb if you want)

MCB InstantaneousTrip current
Type (0.1-5s)

1 2.7 to 4In
B 3.0 to 5In
2 4.0 to 7In
C 5.0 to 10In
3 7.0 to 10In
4 10.0 to 50In
D 10.0 to 20In

Alternatively contact manufacturer especially where you have old type 4's.

Regards
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