Higher Continuous Operating Voltage, Lower Breaking Capacity of Circuit Breaker

What sort of arc do you want to achieve, Guv?

A big one by the looks of the question.

At the risk of answering homework, a magnetically tripped breaker is designed for a specific voltage and current. Increasing the voltage across the sensing element will increase the magnetic field and cause the breaker to trip at lower current levels.

With power constant, as voltage goes up current goes down. Its the whole point of the transmission grid, step up the voltage so the cables can be thinner = cheaper.

Lower available current means lower prospective short circuit current. Circuit breakers are designed to handle the expected short circuit current rather than being over engineered to handle currents they would never see, again to save money.

Dear Jignesh, Hope you are my good ol' colleague from L & T. Hope you could identify me.

Now, arc is not current, but energy (I2rt) or let us say, power. Thus, the breaking capacity, in fact, is MVA. For a given breaker design, this breaking capacity MVA is constant. When MVA is constant, naturally, when voltage increases, current obviously, has to reduce.

Hope it is clear now.

I don't think that it should be related to MVA (Voltage & Current). My question was that generally for eg, a circuit breaker as breaking capacity of 50kA@415V AC then why does it reduce say for eg 36kA@690V AC. It is my opinion that it cannot be just derived by cross multiplication but may have some relevant technical answer for reduced breaking capacity.

It is a combination of issues. Mostly it is the energy issue raised by Eletricalexpert, because the energy in the fault translates directly to mechanical (magnetic attraction and repulsion) forces that the breaker must withstand without becoming shrapnel.

But there is also an element of higher voltage helping to sustain an arc longer. That's why the arc chutes on a 5kV air breaker are VASTLY different than the arc chutes on a 600V breaker. The difference between 415V and 690V are not as great, but it does make for the relationship of the MVA to appear non-linear.

Dear Jignesh,

Yes! It is related to MVA. But, as put forth by Mr. JRafe, the relationship is not linear (i.e.) the current does not linearly reduce in proportion to the voltage rise. So, the question of cross multiplication does not arise. I haven't said so either.

Also, at higher voltages, the voltage across the contacts is also more and thus the restriking voltage. Hence, the breaker cannot have the same breaking capacity at higher voltage as the magnitude of the restriking voltage also is more.