An MCB is used to protect the cable from damage due to overload and short-circuit currents by disconnecting the supply within a certain time.
The time in which it disconnects the supply is dependent on the amount of current that is flowing.
This is why we have maximum Zs values, so that in the event of a fault to earth, enough current will flow to operate the MCB within the specified time. (0.4 seconds for circuits ≤ 32A and 5 seconds for circuits > 32A)
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An RCD is used as additional protection against electric shock by detecting earth leakage (imbalance between line & neutral) but it will not detect a short-circuit between line & neutral.
17th Edition currently requires the use of RCDs for
- socket outlets not exceeding 20A for use by ordinary persons except where the socket outlet is labelled for a particular item of equipment
- mobile equipment with a current rating not exceeding 32A for use outdoors
- all circuits installed in a Special Location
- cables concealed in walls < 50mm and not mechanically protected
An RCD will operate much faster and at much lower fault currents than an MCB and usually protects multiple circuits
1 x IΔn = 300mS
5 x IΔn = 40mS (≤ 30mA only)
IΔn = RCD operating current (e.g. 30mA)
RCDs are also used in TT installations where the Zs of circuits is too high for the MCBs to operate in the specified times.
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An RCBO combines the 2 above. If RCBOs are used there is no need for them to be protected by RCDs