The Residual Current Device, or RCD, is one of the most important electrical safety devices in both the home and commercial environments. If the RCD device has additional overcurrent protection integrated in the same device, it is referred to as RCBO.
In short, an RCD protects against injury and death caused by electric shock by detecting an imbalance between the outgoing and incoming total current of a given number of circuits. It works on the very simple principle that if there is a difference in the electricity coming in against that which is going out, then it must have gone somewhere.
This phenomenon is commonly known as 'earth leakage' - (or the 'Residual Current' in the RCD's title). It can be caused by a number of problems including poor electrical wiring and DIY accidents such as drilling through a cable when mounting a picture hook or cutting through a cable with the lawn mower. In this type of situation, the electricity must go somewhere and choosing the easiest route can travel through the lawnmower or drill to the human holding them to create a circuit and cause electric shock.
An electrical current of just a few milliamps (mA) experienced for just one second is enough to kill most fit, healthy people. RCDs therefore have two key aspects to their operation - the amount of current they allow for Earth Leakage before operating - the mA rating - and the speed with which they operate - the ms rating.
In the UK, standard domestic RCDs operate at 30mA. In other words, they will allow a current imbalance below this level in order to account for real world situations and avoid 'nuisance tripping’ but will cut power as soon as they detect a current leakage of 30mA or above. UK regulation BS EN 61008 stipulates that RCDs must trip within certain time frames depending on the amount of current imbalance.
1 x In = 300ms
2 x In = 150ms
5 x In = 40ms
'In' is the symbol given to tripping current - so, for example, 2 x In of 30mA = 60mA. RCDs used in commercial and industrial environments have higher mA ratings of 100mA, 300mA and 500mA.
