Recommended 40A MCB trips

[BR]

Agree, BS7671 charts show a touch over 100amp to trip a B40 trip in 60seconds. BUT in reality it would happen ALOT quicker.

As far as I can remember, a hager B40 MCB running as 106A will trip after approximately 3.5secs. Installed individually (no de rating factors applied) in a 35oC environment.

But there are many other issues such as grouping factors which should be taken into consideration. I can't comment on BS7671 charts as we don't use them in Ireland, However in practice the above example would be more typical.

 

[altec]

do you mean that having more than one MCB installed alongside each other in a fuse board de rates it???

 

[BR]

Yes, circuit breakers are designed and calibrated to carry their rated
current and to operate within their designated thermal time/
current zone at 30oC (or occasionally 40oC). Testing is carried out with the
breaker mounted singly in a vertical plane in a controlled
environment. Therefore if the circuit breaker is required to
operate in an environment that differs from the reference
conditions, certain factors have to be applied to the standard
data. Where a circuit breaker is required to operate at a higher
ambient temperature it will require progressively less current to
trip within the designated time/current zone.

For example a B40 MCB operating in an enclosure with an ambient temperature of 60oC will is de rated to 25A however some overload characteristics can be applied after this.

Grouping factors
Consideration should also be given to the proximity heating
effect of the breakers themselves when fully loaded and
mounted together in groups. Depending on the trip rating the
watts loss from each breaker may well raise the ambient air
temperature of the breaker above the ambient air temperature of
the enclosure.
Grouping factor (rated current reduce by factor K)

no. of units n K

n = 1 1
2 &#8804; n < 4 0.95
4 &#8804; n < 6 0.9
6 &#8804; n 0.85



Example
Five circuit breakers are to be installed inside an enclosure in a
switch-room, which has an average ambient air temperature of
35ºC. Each circuit breaker will be required to supply a
continuous current of 20A.
From the table below a circuit breaker would be selected with a rated
current of 25A at 30ºC and 23.5A at 35ºC. This takes care of the
switch-room ambient air temperature of 35ºC, but the grouping
factor of five continuously loaded breakers mounted together in
one enclosure must also be taken into account. Table 5 gives a
grouping factor K of 0.9. This grouping factor is then applied to
the rated current at 35ºC giving a circuit breaker rated current of
23.5 x 0.9 = 21.15A in the specified conditions.

Table linked to below as would not display properly here.

http://homepage.eircom.net/~bryanrooney/