Electrical Guides
February 27, 2023

What to know about RCD’s

A RCD (residual current device) is an electrical protective device, usually found within your switchboard. Sometimes you’ll find them built-in to a socket outlet, or as part of an extension lead. RCD’s protect you against electric shock and protect your property from fire by automatically disconnecting the circuit when it detects current leakage. 

What do RCD’s do? 

A RCD (residual current device) is an electrical protective device, usually found within your switchboard. Sometimes you’ll find them built-in to a socket outlet or as part of an extension lead. RCD’s protect you against electric shock and protect your property from fire by automatically disconnecting the circuit when it detects current leakage. 

Why is my RCD tripping? 

There are many different causes for RCD tripping. The most likely cause of RCD tripping is a faulty appliance. Another common cause of RCD tripping is outdoor electrical fittings (sensors, lights, outdoor outlets etc). These fittings can become waterlogged,  which will cause current leakage as water to come into contact with live parts within the fitting.

If your RCD has started tripping since you’ve done some water-blasting, or after heavy rain then this is the likely cause. 

The RCD itself can also malfunction and start ‘nuisance’ tripping. If you’re unsure what I’ve us a call or email if you experience any of the above. Our team will diagnose the issue and get your electricity restored quickly. 

What can I do to fix it? 

If your RCD tripped after you turned something on (or plugged something in): 

It's highly likely that the appliance you plugged in is faulty and is causing the RCD to trip. 
Disconnect the appliance completely, then reset the RCD to check. 

If the RCD doesn’t trip: You have a faulty appliance.  Give us a call to check if we can repair it. 

If the RCD trips again: then the problem is elsewhere – follow the steps in the next section

If your RCD tripped randomly: 

Leave the RCD off, then walk around your property and test all appliances that are plugged in. 
If an appliance is plugged in and still has power / is operating, you can rule it out as the cause. 

Unplug every device that is plugged in and won’t turn on. 

Once you’ve done this, go back to the switchboard and switch back on the RCD. 

If the RCD doesn’t trip:
Great! - Start plugging your appliances in again, one-by-one.  If you plug something in and it trips the RCD, you have found your faulty appliance. Give us a call to check if we can repair it. 

If the RCD trips straight away:
Then the problem is elsewhere – get in touch with one of our friendly team, we’ll book in a time to stop by and investigate. 

How do RCD’s work? 

Simple explanation: 
Imagine your toaster has an electrical issue, and the outside metal casing is LIVE. 

If you touch the toaster, you’ll get an electric shock. 
Current will be flowing through your hand and arm, through your torso, and down your legs and into the ground.

A RCD detects this shock current, and rapidly disconnects the supply of electricity – saving your life. 
RCD’s are designed to operate within 30milliseconds - before ventricular fibrillation (a heart attack) can occur.

Electrical explanation: 
Within the RCD there is an electronic circuit which monitors the balance between the currents running through the phase conductor (or conductors) and the neutral conductor. 
During normal operation, the currents will be balanced. 

When an earth fault occurs, some of the current from either the neutral or the phase conductors will travel via the earth parallel path, back to the MEN points of the electrical system. 

This creates a current imbalance within the RCD.

The imbalance causes an induced current in a toroidal coil, which is amplified within the RCD’s internal electronics, actuating contacts and disconnecting the supply. 

What causes a RCD to trip? 

A RCD will trip when it detects a current imbalance in the live conductors of a circuit. 
A current imbalance typically occurs when one of the live conductors comes into contact with earth.

The connection to earth can be through conductive building materials, or worse – through you! (electric shock).
When this happens, some of the normal circuit current will flow through earth. 

A RCD detects this fault and automatically disconnects power to the circuit, preventing property damage and physical harm.

How does a RCD prevent electric shock? 

If you come into contact with a live electrical conductor, current will flow from the point of connection - through your body - to the earth (electric shock).

As long as the shock current value is larger than 30mA (milliamps), the RCD is designed to detect it and automatically disconnect the supply, preventing further shock. 

Why 30mA? because any shock current above 50mA can cause cardiac arrest (heart attack) in an adult human. 

Why do both the lights and power turn off when the RCD trips? 

In NZ it is common for electricians to install a single RCD to protect multiple circuits – typically 2 power circuits and 1 lighting circuit (3 circuits per RCD maximum).

This is a cost-effective way to provide RCD protection, however it means that if there is a leakage fault on a power circuit, your lighting and other power circuits may also be affected. 

Installing one RCD per circuit is more expensive initially, but will prevent these types of issues occurring. 

Can an RCD trip with no earth conductor? 

Yes – even if your circuit cables don’t have an earth conductor, current can flow to earth through any conductive materials that the live conductors come into contact with. 

Is an RCD the same as a circuit breaker? 

No – a RCD and a circuit breaker each have their own protective function.
A RCD detects current imbalances, whereas a circuit breaker detects current overloads & short circuits. 

Both devices work by disconnecting the circuit when their respective faults are detected. 

Will an RCD trip on overload? 

No – an RCD (or RCCB) will not trip if there is an overload on the active conductors, it only detects current imbalances.
For this reason they must be combined with MCB’s or alternative overload protection. 

RCBO’s are devices which combine both MCB (overload) and RCD (residual current) protection into one. 

Is an RCD a legal requirement? 

Yes – in NZ, RCD’s are legally required to be installed on residential installations where new electrical work or additions have been carried out.  There are a few exceptions to this rule, but in general it’s always a good idea to have them installed. 

Schools, childcare centres and patient areas within medical facilities are required to have specific RCDs installed for a greater level of protection. 
RCD’s are not typically required in commercial and industrial settings, however it is good practice to have them installed on circuits containing general purpose outlets used by staff or the public. 

Get in touch with us if you’re unsure what level of protection you require for your installation. 

How do I test that a RCD is working? 

RCD’s have a test button which will trip the RCD when pressed, disconnecting the circuit. 

It’s a good idea to test your RCD’s regularly to determine that they are still operational (every 6 months is a good rule of thumb). 

During installation our electricians will perform comprehensive RCD checks & tests, to ensure that each RCD meets all the requirements set out in the NZ electrical regulations. 

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