As per my knowledge, the overload current will trip a circuit breaker and will burn the fuse of any fused switch. How come when my main breaker failed (because of the steamer I reckon because it happens every time we use it) the fuse of the switch that held steamer socket + the ELCB didn't trip (It jumped directly to the MCB)?


"Circuit breaker coordination" is the term used when setting up multiple breakers / fuses. The trip curves overlap and in many cases a small breaker can take longer to trip than a large breaker.

One example I can provide is this: the plant I work in was not built with the same brand breakers. In one area an 800 amp main will trip out prior to the 400 amp mcc feeder or the 100 amp breaker on the motor overload bucket. The cost to coordinate these breakers to the correct ones would be several thousand dollars and it happens so infrequently that the owner is OK with the occasional down time. By comparing the curves of the different types of breakers you can see that fuses and small inverse time breakers will hold at 4-6 times their rated value for a short time.

So depending on the loading of the circuit and type of breakers used, if not well coordinated, the main will open before a much smaller breaker protecting the wiring but dropping power to everything. A well coordinated system takes planning and the correct breakers and fuses to prevent this from happening.

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    Explains a situation I had last year. Yard workers hit the 30 amp lines to my shed with a trencher. Popped the breaker to the house, not the shed. – Machavity Apr 12 '17 at 14:49
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    I know zilch compared to the professionals on this site, but, Machavity, if the trencher shorted the lines from the house (where the panel is) to the shed, this presumably would not result in a high current in the cable between the damage point and the shed. On the other hand, if this damage point was beyond the shed, say to a line from the shed to an outdoor receptacle, then this would be different. – Jim Stewart Apr 12 '17 at 18:18
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    @Jim stewart, I believe the 30 amp that supplies the shed breaker did not trip but the main did, This would not surprise me with a short. As I posted I have seen a 800 trip without the smaller breakers tripping and the short was protected by a 100 amp breaker. Total load on this feeder is close to 400 amps when everything is running. So there is tons of head room. – Ed Beal Apr 12 '17 at 18:55

It's very typical for the MCB to also be an RCD, and perhaps the device failed with a ground fault condition, causing an RCD trip rather than an excess current trip.

So are you going to get rid of that steamer, or what? If a device trips a circuit breaker everytime you use it, stop doing that! When a breaker trips frequently, that ages the breaker, that thing may not trip one day, and then, you'll trip instead! Or one of your family.

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  • If a given appliance has a leak to ground, will that be revealed with an ohmmeter between the hot and the ground or the hot and the case? Can an appliance like a steamer leak current to a floor or a wall sufficient to trip a RCB? – Jim Stewart Apr 12 '17 at 18:27
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    @Jim that depends if the fault chooses to cooperate. Many faults are like VBO devices, they don't flow below a certain voltage, so a DVM may not reveal them. – Harper - Reinstate Monica Apr 12 '17 at 20:15

LH where are you? Reference to a fused switch indicates to me you are in the UK. I am guessing that a fused switch is a way of connecting a spur to a ring circuit. We don't use that in the US so I am sorta guessing that the current in the ring could be near capacity and so if one connects an additional significant load to a spur (which is below the rating of the spur fuse in the switch) one could trip the main breaker and not burn out the fuse in the connection to the spur.

If this is so, you should be able to operate the steamer on this circuit if you switch off enough other loads before connecting the steamer.

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  • Hi. I am based in Bahrain. We operate with a UK system I believe. – L H May 11 '17 at 5:40
  • How was this resolved? Did you repair or replace the steamer? Or did you discover that there was a problem with your house wiring? – Jim Stewart May 11 '17 at 10:55
  • Actually I have took the steamer to the agent for a repair, didn't hear from them since about 2 weeks, but since then no trips happened! – L H May 21 '17 at 11:03

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