What is a Safety Switch or RCD?
It's a Residual Current Device or Safety Switch and it provides a fast power cut-off in problem situations.
How does an RCD work?
An RCD monitors the current flow in the active and neutral wires of the power lead or device connected to it, all the time looking for an imbalance or difference in the currents in each wire.
The undesirable current imbalance in the active and neutral wires occurs when current flows to earth.
Earth leakage safety switches work on the principle that current going into a device should equal the current exiting the device and that any difference between the two is due to current flow via an unwanted or undesirable path.
The undesirable current path may be via a person or an internal fault in an appliance or tool. The imbalance may also occur by cutting the power cable or by using tools or appliances in unsafe wet or damp areas. All electrical equipment will have some amount of current flow via earth (earth leakage). It may be minute (< 100 micro Amps); but it must be less than the limits specified in the AS/NZ 3760 standard (1.0 mA for Class II double insulated equipment or 5.0 mA for Class I or earthed appliances).
Safety Switch Types
There are two main safety switch types commonly in use: - Type 1 - used in medical, hospital or patient care situations and Type 2 in construction areas, workplaces and homes.
For Type 2 switches, if an imbalance of 30mA or more occurs, the RCD will trip - usually in less than 30 milli-seconds, but typically anywhere between 8 and 30 milli-seconds with a maximum test time of 300 mill-seconds.
The nominal tripping sensitivity for a Type 2 safety switch is 30mA; but from experience, the measured value is anywhere between 18.5 and 25mA.
If the safety switch or RCD tests properly without any appliances plugged into it, but trips each time an appliance is plugged in, then the appliance has an earth fault or excessive leakage current.
DO NOT BYPASS the SAFETY SWITCH - IT IS PROTECTING YOU FROM ELECTROCUTION.
RCD's do not protect you from direct contact with both active and neutral wires in an electrical circuit, only the Live or Active (nominally Brown ) and Earth.
Usage Rules for Portable RCD's
The RCD should be plugged directly into a power point and NOT an Extension Lead. The extension lead may be faulty and prevent the switch from operating in life threatening situations.
Do Not Use with any lead greater than 40 metres long. This can cause false tripping and frustration when no fault exists.
Test the RCD frequently and before each use to ensure proper operation.
Do not use any appliance or tool in a wet environment.
Do not immerse in any liquid or directly expose to rain.
If the RCD is dropped, do a push button test before using.
RCD's are designed as safety devices - do not use them as On-Off switches.
Electricity is a wonderful slave but a very dangerous master and the use of a safety switch is never a substitute for basic safety precautions.