RCCB/MCCB/MCB related question 3

[Cromr]

hi all,

i'm quite new to electrical engineering field...particularly related to building wiring, switchgear, switchboard, circuit breakers, etc
not sure how to ask this question but here goes...

i've been looking at some electrical drawing of Main Switch Board and i notice there MCCB and RCCB (residual current circuit breaker) with various ratings 63A, 40A, etc

i understand that this values govern the amout of current that is allowed into the MSB. However, i also notice that there a smaller value of current which is related to them. This values is in milli Amps (e.g. 30mA, 100mA)
i was told that this is the trip current (the amout of current that would trip the circuit) so my question is as follows
1. how does this trip current come about?
2. when we see a RCCB with say 63A rating with 100mA trip current, is this 100mA the amout of current that that exceeds the 63A rating (intake) or is it the current that is present in the neutral line when a short occurs?
3. i also heard that there is such a thing as earth leakage current, is this the current that leaks through the earth line from the system?

as you can see i'm quite at a loss here...
sorry if my question seems to novice but i would really hope that someone could clerify it for me.

thanks in advance for you help

Regards,
Cromr

 

[UKEng]

In simple terms a residual Current Circuit Breaker works as follows:
The current 'going out' to the load on the phase conductor is compared with the current 'coming back' on the neutral conductor.
In a healthy circuit these currents are equal. However if the circuit has an earth fault there will be a differnce in these values. If the difference is greater than the trip rating (100mA in your case) then it will trip.
The 63A rating is the continuous maximum load it can carry.
You can get a device which is a combined MCCB/RCCB which will protect against short circuits, overloads and earth leakage.

This is basic but hope it sets you off in the right direction.

 

[CraigD]

Chunky is right on the money. Perhaps I can expand a little more. If we are talking about the combined residual current device (rcd) and mcb then there are actually 3 types of trips that can occur for different reasons. The first type is the residual current trip where the device measures the active and neutral currents and trips if they are different (indicating that there is a leak to earth somwhere) by the amount (mA) indicated on the device. This can be done a couple of ways but the simplest method is by running the active and neutral conductors through a tiny current transformer. The secondary voltage will be zero if there is no leakage because they cancel out. Generally the devices rated at 10mA are used for hospitals and circuits that require high protection - but with this high sensitivity also comes a greater risk of nuisance tripping. Domestic situations generally require 30mA protection and industrial 100mA. The second kind of trip is the overload trip which is typically based on a bimetal that is in series with the load and heats up and bends far enough when overloaded to cause a trip. The third type of trip is the short circuit trip which is pretty much instantaneous and is tyically based on a solenoid action for currents much larger than the load current (about 100 times rated to give you an idea). You will find on your breakers another value which represent the kA rating that the breaker can handle without causing damage to itself in one or two short circuits. These values are typically 4.5kA, 6kA, 10kA, etc. When a short circuit occurs, the solenoid plunger is accelerated forward to hit the mechanism which trips the breaker. Hope this helps.