Switch the circuits, or the busbar ?

[SimonH2]

Well it should never be called upon to break a fault current, but if a fault developed while the building was empty then it could be asked to close onto a fault. However I'm struggling to think of a fault that would develop while the power was off (apart from things like someone sticking a nail through a cable), rather than developing (eg an arc striking in a mouse chewed cable) after the power is applied.
Considering the current profile when starting a large motor, I don't think there's much to worry about.

 

[JohnW2]

Well it should never be called upon to break a fault current, but if a fault developed while the building was empty then it could be asked to close onto a fault.

Well, for a start, I think you really are scraping the barrel to an unnecessary extent As you go on to say ...

However I'm struggling to think of a fault that would develop while the power was off (apart from things like someone sticking a nail through a cable), rather than developing (eg an arc striking in a mouse chewed cable) after the power is applied.

Indeed.

In any event, I don't think the "closing onto a fault" is really a problem, anyway - it is breaking very high currents which can cause problems.

Kind Regards, John

 

[bernardgreen]

Well it should never be called upon to break a fault current,

MCBs are called on to break a fault ( as opposed to overload ) current and are seldom replaced after the event.

Contactors that have opened or closed on to a fault would often be replaced at the same time as the fault wa being repaired if there was any sign of damage to the contact pads.

 

[SUNRAY]

Contactors that have opened or closed on to a fault would often be replaced at the same time as the fault wa being repaired if there was any sign of damage to the contact pads.

Indeed. several years back I was asked to unwire and later rewire a chillers compressor motor which had stopped working. I found the MCB had been reset several times during faultfinding which was diagnosed as a winding shorted shorted to earth.
I came armed with replacement contactors and MCB when it had been rewound, the aircon guys didn't understand why.

 

[bernardgreen]

A question from the past.

 

[JohnW2]

MCBs are called on to break a fault ( as opposed to overload ) current and are seldom replaced after the event. .... Contactors that have opened or closed on to a fault would often be replaced at the same time as the fault wa being repaired if there was any sign of damage to the contact pads.

In wondering whether there is a rational basis for that difference, I wonder whether it is the 'arc chamber' etc. present in MCBs?

Kind Regards, John

 

[SimonH2]

That's a bit of a random diversion
My answer would be that there's very little in it, but the lower option will have a very very slight edge.
At switch on, the resistor will limit the current pulse as the capacitance of the cable charges up. As a secondary effect, if the combination of source resistance and cable capacitance reduces the initial current, it will also very very very slightly reduce the current drawn by the lamp when it first turns on. Once capacitance charged and lamp hot, location of the resistor makes no difference.
As to what happens at switch off, well that very much depends on the characteristics of the cable and value of resistance. If there is any voltage spike due to interrupting the current (due to inductance of the cable), then the resistor will limit the current flowing through an arc as the relay contacts open.
And if someone sticks a nail through the cable, then the resistor will control the fault current - and if suitably rated would make the circuit safe from faults. Suitable rating and locations of local indicator lamps could differentiate between off, on and no fault, on and short circuit cable, on and open circuit. A single indicator lamp replacing the resistor could differentiate 3 states but would not differentiate between off and open circuit.

 

[bernardgreen]

but the lower option will have a very very slight edge

Experience from bygone days is that the advantage is more than just slight.

 

[SimonH2]

In wondering whether there is a rational basis for that difference, I wonder whether it is the 'arc chamber' etc. present in MCBs?

I suspect the only difference is that the contacts are typically easily accessible for inspection on a contactor - and not at all accessible on an MCB. So if the MCB still appears to work - supplies the load when on, switches off properly with the operating lever - then we assume it still works. With the contactor we can look at the contacts and if they look bad it gets replaced, even if it still works as well as the MCB does.

 

[JohnW2]

I suspect the only difference is that the contacts are typically easily accessible for inspection on a contactor - and not at all accessible on an MCB.

That might have been true in the fairly distant past, but (at least at 'domestic' current levels) I think you'd now struggle to find (m)any whose contacts were any more accessible for inspection than in the case with MCBs, since they seem to come in essentially 'MCB-like' cases. See how many you can find out of all these which are likely to have 'easily inspectable' contacts .... (click here) !!

Kind Regards, John