Motor Circuit Breaker Earth Continuity

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JamieP
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Motor Circuit Breaker Earth Continuity

Post by JamieP »

First time I've had to test circuits with motor circuit breakers and just curious how to calculate EC values for the circuit?

Is it just a case of getting some manufacturers data and calculating for a 5s trip time? Or are they similar to standard circuit breakers and should I be aiming for the instantaneous zone?

Still learning and trying to understand how they work and what's actually different but definitely wanting to make sure I at least get the testing right
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gregmcc
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Re: Motor Circuit Breaker Earth Continuity

Post by gregmcc »

I'll start this with...."Correct me if i'm wrong"

There are a lot more things to consider on a motor circuit breaker compared to a regular circuit breaker.
Some have adjustable trip time setting (some don't).
So the way I understand it should work is, if the circuit is for personal protection then the trip time must be no greater than 0.4s, if it's for machinery (like a motor) then it's 5s, table 8.1 in 3000 does not show 5s trip times for circuit breakers.

I think you are on the right track on getting manufacturers data, a good starting point for example for a 16-20A motor circuit breaker would be to use column Type D, 20A row

the difference between the 2 is that Motor Circuit breakers have auxally contacts than may indicate open or closed or tripped and usually have an adjustable trip range, other than that they are the same as a circuit breaker
JamieP
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Re: Motor Circuit Breaker Earth Continuity

Post by JamieP »

Thanks Greg, I understand how to calculate the value and such but it's more if there are other things I need to consider as said due to not having dealt with them and also not fully understanding what's different, appreciate the input
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Re: Motor Circuit Breaker Earth Continuity

Post by AlecK »

Since you're looking at EC values; we must be talking about fault protection.

True that Table 8.1 doesn't show EC values for 5s for circuit breakers.
That's because the info simply isn't needed.
It's needed for fuses; because they are purely thermal devices.
But for circuit breakers the fast-operating caused by high fault currents - ie both fault protection & short circuit protection - is magnetic.
So pretty much regardless of the actual curve, the operating time will be well under 5 sec.

Greg has suggested that a max operating time of 0.4 sec would apply for 'personal protection"; which is a reasonable approximation.
However (being pedantic) the requirement isn't actually set WRT "personal protection"; it's set for circuits supplying socket outlets.
It's very unlikely a motor circuit breaker would be used for such a circuit.
They're much too expensive to be wasted like that.
But if that rare circumstance occurs, then you would need manufacturer's data to determine what the exact max EC value (for the particular load current setting) is.
As Greg suggests, they are typically D-curve; and if you use the highest setting as nominal rating you can't really go wrong.
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