Also, RCDs/RCBOs are not fail-safe devices. If the electronic circuitry inside stops work (as happens sometimes), they generally stay switched on and fail to trip when required. Relying on one as the primary means of protection would thus be rather foolish. Remember to test them from time to time too!
The RCCD triggers if there's a current imbalance, which requires the difference in current to pass through some earth connection somewhere. One of the functions of a deliberate earth connection is to provide this (thus tripping the RCCD at the time the faults occurs) rather than having to wait for you to come along personally and stick a fork in the toaster, earthing it through yourself.
I prefer my faulty appliances disconnected early, without having to use myself as a current path.
Incorrect statement: the current path frequently is through the circuit earth wire (properly called circuit protective conductor). Indeed in TN-earthed installations it's recommended that disconnection in the event of an earth fault to an exposed-conductive-part should not rely on the presence of any RCD in the circuit. IOW the earth fault loop impedance (Zs) should be low enough to ensure that the overcurrent protective device (fuse or MCB) operates within the relevant permitted disconnection time. Fault protection is then maintained, even if the RCD is faulty. The RCD provides additional protection against the less common situation where the current path is through someone's body.
No. For example a wire breaking loose in a metal lamp fitting (not uncommon) on an earthed circuit would cause an immediate disconnection of the supply, and an inability to reset the MCB while the fault is present. The RCD acting on its own would only be able to offer any protection when you actually got a shock, rather than before.
That is the point. An RCD is supposed to cut off so quick you will not get a fatal shock. If that is the case then all this earthing wiring is just backup if the RCD is faulty. When an RCD is fitted that the prime protection mechanism, not the earthing.
The key issues here are really the "supposed to" and the "fatal" bits. The RCD is an inherently more complex device, and there are more reasons why it may fail to operate (not only due to its being faulty). Secondly, many faults in an earthed system will be cleared without anyone receiving a shock. While the RCD *should* render a shock non fatal (assuming its a 30mA trip device or lower), that does not mean it is always non fatal, and it also does not mean you will escape injury. That could be directly - pain, localised burns, maybe even arc flash injuries in some extreme cases, or consequential - recoiling and bruising / lacerating yourself, falling etc.
Its the first acting protection in the minority of typical fault conditions.
I am not denigrating RCDs in any way - they do a very worthwhile job and are certainly an added level of protection in a number of cases if used properly. However, personally I would want to rely only on them as the only protection.
(its rather a moot point really given the regulatory requirements)
Say you have some appliance with earthed metalwork - I chose the example of a metal lamp fitting. Many of these will have the bulb holders wired in fairly flimsy wire that is vulnerable to being twisted and pulled from its terminations when someone ends up screwing round the whole bulb fitting trying to free a stuck bulb.
Should a live wire come free and make contact with the metalwork, this would cause a large current surge to flow to earth next time you turn the lamp on. This will blow the fuse or open the circuit breaker. If you happened to be changing the bulb while the fitting is powered, the earthing would also limit the voltage rise on the metal (the circuit breaker / fuse / RCD can only limit the time that power remains connected, and hence only the duration of the shock not its magnitude)
With the faulty earthed appliance, the MCB will react any time that power is applied. Hence if you try resetting the MCB it will fail to do so. A RCD however would only usually be responding to a transient event
- say someone touching the appliance. So you could go reset it, and come back for another go.
(you may think that having got a belt off something the once, people would be reluctant to try it again until they have worked out why and fixed it, but that only seems to be the mindset of the engineer or DIY enthusiast!)
With earthing, many faults will be detected and cleared by the fuse / MCB or RCD when they happen. You also have two devices acting in your defence (e.g. MCB and RCD) - you would need a cascade of failures for then to not clear the fault between them. If all you have is the RCD, then a fault may lay in wait for the next unlucky person to complete a circuit. They may well live to tell the tale, but would presumably rather not have had a shock in the first place.
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