RCD AND MCB what can they detect?

[ricicle]

An MCB will operate on overload (ie non-fault), also on short - circuit (L-N or L-E) The time taken to operate is inversely proportional to the overload/short-circuit current (usually 0.1 sec is taken as the fastest operating time due to mechanics for faults of negligible impedance)

An RCD operates quickly for smaller 'out of balance' currents ( ie the difference between L and N) So will most definately operate for a short - circuit between L and E and they usually operate for N to E faults depending on currents flowing in the installation.
The RCD is more sensitive and so is prone to nuisance tripping eg damp in fittings, poor insulation but is also there as additional protection as the small fault current could be flowing through you !
40mS is the required max. trip time for this kind of additional protection.

 

[TheOriginalTonkaToy]

so to finally finish it off

A RCD can detect live to earth,neutral to earth, and the MCB can only detect live to neutral faults is that right??

An MCB will also detect live to earth faults if the design current is exceeded.

RCDs protect people, MCBs protect cables.

 

[kevin1234]

thanks very much guys!!

 

[ricicle]

RCDs protect people, MCBs protect cables.

MCBs also protect people !

 

[TheOriginalTonkaToy]

RCDs protect people, MCBs protect cables.

MCBs also protect people !

In that they prevent fires, yes. If you're in the s/c path with sufficient current flowing through you to trip the MCB then you're probably dead anyway.

 

[Spark123]

If there is no RCD on a circuit and there is a L-E fault then the MCB needs to disconnect the supply quickly enough to remove danger too.

 

[ricicle]

RCDs protect people, MCBs protect cables.

MCBs also protect people !

In that they prevent fires, yes. If you're in the s/c path with sufficient current flowing through you to trip the MCB then you're probably dead anyway.

They also limit the duration of touch voltages on exposed conductive parts under fault conditions

 

[TheOriginalTonkaToy]

RCDs protect people, MCBs protect cables.

MCBs also protect people !

In that they prevent fires, yes. If you're in the s/c path with sufficient current flowing through you to trip the MCB then you're probably dead anyway.

They also limit the duration of touch voltages on exposed conductive parts under fault conditions

Think about that one. For an MCB to trip you have it exceed its rated current.

 

[ricicle]

Think about that one. For an MCB to trip you have it exceed its rated current.

Yes, but I am talking about shock protection from exposed conductive parts (fault protection) rather than basic protection !

 

[Spark123]

MCBs don't offer any protection against direct contact (basic protection), RCDs can only provide supplementary protection against direct contact. (sorry - 16th edn terms stuck in my head!)

 

[TheOriginalTonkaToy]

MCBs don't offer any protection against direct contact (basic protection), RCDs can only provide supplementary protection against direct contact. (sorry - 16th edn terms stuck in my head!)

Exactly. They serve the same protective purpose as a wire or cartridge fuse - they serve to protect the cable against overcurrent.

 

[Spark123]

MCBs also provide indirect contact protection (fault protection) which is what Ricicle was saying i.e. cut the power quickly enough to remove danger should an exposed conductive part become live under a fault condition.
RCDs also offer indirect contact protection.

 

[TheOriginalTonkaToy]

Yes, they operate if there is a low resistance fault path to earth and remove the touch voltage risk but it's the local earth that provides the protection by causing the device to operate. If a person provides that fault path themselves then I'm afraid the MCB isn't going to be much help.

I think we're saying the same thing in different ways...

 

[Spark123]

Yep, an MCB doesn't provide any supplementary protection against direct contact.