RCD AND MCB what can they detect?
- Thread starter kevin1234
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An MCB is an overcurrent device. It acts just like a fuse.
An RCD detects the difference between the current flowing on the live and neutral, and trips if it exceeds a set value.
An RCD detects the difference between the current flowing on the live and neutral, and trips if it exceeds a set value.
An MCB is a Miniature Circuit breaker. These normally have two parts, a thermal and a magnetic part. The thermal part causes the breaker to trip if it is slightly overloaded over a long period of time (i.e. it heats up, when it gets to a certain point it causes the breaker to trip). The magnetic part detects very large overloads, such as a live to earth fault.
An RCD is a Residual Current Device. This monitors the current going out on the live, and coming back on the neutral, and makes sure they're equal. If they differ by more than the rating of the device (commonly 30mA and 100mA ones), it trips. This will therefore detect a live to earth fault much more quickly, as it only requires 30/100mA to flow, unlike an MCB which will require enough current to trip the magnetic part to flow.
One thing to mention is that an RCD will not detect an overload, it only monitors for earth leakage. You can get RCBOs, which is a single breaker that combines an MCB and RCD.
An RCD is a Residual Current Device. This monitors the current going out on the live, and coming back on the neutral, and makes sure they're equal. If they differ by more than the rating of the device (commonly 30mA and 100mA ones), it trips. This will therefore detect a live to earth fault much more quickly, as it only requires 30/100mA to flow, unlike an MCB which will require enough current to trip the magnetic part to flow.
One thing to mention is that an RCD will not detect an overload, it only monitors for earth leakage. You can get RCBOs, which is a single breaker that combines an MCB and RCD.
so i have been seeing on websites it says that the RCD will not detect any live to earth, so i assuming that is wrong then, it can detect them is that right? it just cant detect overloads is that right?
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A live to earth fault will cause an RCD to trip - that's the main reason for having them, in case the fault path is through a person.
Residual current detection is complementary to over-current detection. Residual current detection cannot provide protection for overload or short-circuit currents.
why does the enyclopedia say that it cant detect short circuits and you guys are saying it can detect live to earth faults?
I also cant find about RCD detecting Neutral to earth faults can they detect it?
why does the enyclopedia say that it cant detect short circuits and you guys are saying it can detect live to earth faults?
I also cant find about RCD detecting Neutral to earth faults can they detect it?
AFAIK...
a) Yes
b) No
RCD detects any difference in the current flow between live and neutral. If you get a neutral to earth fault *blip* the RCD should go. Live to Earth again the RCD should go. Live to Neutral no - the same current will be flowing in both live and neutral and will not register. All subject to me understanding it correctly.
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Pobably because by short circuit it means live to neutral which is the one thing RCDs dont do (thats what the MCB or Fuse does).
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Almost. An RCD will also detect live to earth only it'll do it a _lot_ slower and need a lot more current than an RCD. The RCD is designed, if properly installed, to trip within a few milliseconds and with a tiny amount of current - quick enough hopefully to save someones life, the MCB will trip relatively slowly and take many amps to trip.
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