Student BS60898 Type B trip question

Jonathan Harris · Nov 12, 2015

Hello

I'm a Level 2 electrical installations student and I have a question about Type B BS60898 circuit breakers.

I've been revising tonight and have noticed that the trip time on these is 3 to 5 times the rated current. What does this mean?

Say I had a 32A Type B would it trip over any current over 32A or would it trip at 32 x 3 = 96A as this is the three times the rated current?

Thanks

Jonathan

Buckfast · Nov 12, 2015

Jonathan Harris said:
Hello

I'm a Level 2 electrical installations student and I have a question about Type B BS60898 circuit breakers.

I've been revising tonight and have noticed that the trip time on these is 3 to 5 times the rated current. What does this mean?

Say I had a 32A Type B would it trip over any current over 32A or would it trip at 32 x 3 = 96A as this is the three times the rated current?

Thanks

Jonathan

Yes you are correct. A 32A Type B miniature circuit breaker should not trip in less than 0.1 second and not more than 5 seconds with 3*In. A current equal to 5*In should trip the Mcb in less than 0.1 seconds.

Regards

Buckfast

Jonathan Harris · Nov 12, 2015

Eodneil said:
The second part mate so 3-5 x 32A I tend to go worst case scenario and do 5 x 32 = 160A, so it will disconnect within 0.4s (TN) at 160A. This is pretty decent for a level 2 enquiry though.

Thanks for the reply. So a type B won't trip unless it is over 3 times the value of the current on the breaker?

telectrix · Nov 12, 2015

it will trip, but not as quickly. e.g. it will take 200 secs. @ 60A to trip. the larger the current, the quicker it will trip.

Jonathan Harris · Nov 12, 2015

Thanks to all!

So it trips quicker if the current is 3-5 times. One last question.

If I have a shower that is rated 8.5Kw Ib= 8500/230= 37A. How is a 32A type B suitable? Is it because the shower won't draw the full load? And if so, why won't it?

Thanks

telectrix · Nov 12, 2015

MCB will stand a small overcurrent almost indefinitely.

Buckfast · Nov 12, 2015

Jonathan Harris said:
Thanks to all!

So it trips quicker if the current is 3-5 times. One last question.

If I have a shower that is rated 8.5Kw Ib= 8500/230= 37A. How is a 32A type B suitable? Is it because the shower won't draw the full load? And if so, why won't it?

Thanks

I would say 32A Mcb not suitable. A 40A would be more appropriate. With regard trip times, The larger the overload/fault current the quicker the Mcb will trip

happysteve · Nov 13, 2015

433.1 specifically precludes the example of using a 32A breaker for a 37A load: "Every circuit shall be designed so that a small overload of long duration is unlikely to occur."

More specifically. 433.1.1 (i): "The rated current or current setting of the protective device (In) is not less than the design current (Ib) of the circuit."

Looking at Fig 3A4 in the regs (p 325), looks like the 32A breaker might never trip at currents upto about 40A (ish, maybe a bit more). But the regs above say you shouldn't do this.