Since the introduction of mandatory installation of RCDs in all new homes, there has been a huge
increase in the availability of these products. There is no doubt that RCDs
increase overall safety when using mains power and are a vital piece of
equipment when using power tools. Indeed, under WorkCover regulations, all
tradesmen on building sites are required to use RCDs when running power
tools.
Before we go too far in this article, we should explain what an
RCD is. "RCD" is an acronym for "Residual Current Device". While sometimes
called a "Current-operated Earth leakage device" most people refer to them as a
"safety switch".
They protect the user from electrocution by disconnecting the
240VAC supply if there is current flow to Earth. The RCD does this by monitoring
the current flow between the Active and Neutral lines for the 240VAC mains
supply. In a normally operating appliance there is current flow from the Active
lead through the appliance and then back through the Neutral lead. This is shown
in Fig.1a. Both currents i1 and i2 should be exactly the same value.
Features
- Suitable for home use, tradesmen, electricians, musicians, etc
- Creates an Earth leakage current to test RCD
- Push to test operation
- Neon indicator shows presence of power and checks internal components
- Neon indicator to check Earth connection and Active/Neutral terminals
- Suitable for type II RCDs (10-30mA rated residual current)
- Applies residual test current from 32.5mA at 192VAC to 45.5mA at 264VAC.
- Complies with Australian Standard AS 3190-1994 for RCD to trip at or above rated residual current of 100% +5% -0% between 80% of rated supply to 110% of rated supply (240VAC).
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If there is a fault in the appliance, as shown in Fig.1b, some
of the current flowing from the Active lead may flow to Earth (i3) instead of to
Neutral. If this is the case it may be that either there is a breakdown in the
appliance between Active and Earth or someone is conducting current away from
Neutral via their body. In other words they are suffering electrocution.
The RCD detects this difference between the Active and Neutral
current and disconnects the power should the difference reach a predetermined
level. The difference between the Active and Neutral current is called the
residual.
An RCD cannot protect against electrocution if the current flow
through the person is between Active and Neutral. This is because the RCD cannot
differentiate between appliance current and current through your body. (The
moral of this is never to work on an appliance which is connected to the 240VAC
mains supply!)
Note that any appliance with exposed metal parts can become an
electrocution hazard and this includes many appliances which are labelled as
"double insulated". All that has to happen is a for a leakage path to develop
between the Active mains terminal in the equipment and the exposed metal parts;
ie, usually the case. Then, if the case is not earthed, it will be live and a
potential cause of electrocution if a person touches it.
There are several types of RCDs. Type I RCDs have a rated
residual current of 10mA and a 40ms tripping time. Type II have a rated residual
current of between 10 and 30mA and 40ms tripping time at 500% residual
over-current. It takes some 300ms to trip at the rated residual current.
Type III RCDs have a rated residual current of between 30 and
300mA and a 50ms tripping time at the 500% residual current. Finally, type IV
has type III current characteristics with selectable tripping times. Type II
RCDs are the most frequently encountered.
