What an RCD is and why it matters
A residual current device (RCD) continuously monitors the current flowing through a circuit. In a healthy circuit, the current going out through the active is matched by the current coming back through the neutral. When some of that current leaks to earth — through damaged insulation, a wet surface, or a person — the RCD detects the imbalance and disconnects the supply. It does this quickly enough that, in many situations, a person would otherwise have received a fatal electric shock.
That is the whole point of an RCD. It is not there to protect wiring or equipment; it is there to protect people. A fuse or circuit breaker protects the cable, and will happily allow a small leakage current to continue if it sits below its rating. An RCD is designed to catch exactly that small, dangerous leakage.
RCD protection is a separate control from testing and tagging
This is the point New Zealand business owners most often miss. Testing and tagging gives you a snapshot of whether an appliance was safe at the moment it was checked. It does not, and cannot, provide ongoing protection while the appliance is in use. RCD protection is a different layer of control.
WorkSafe NZ is clear on this: whether equipment has been tested and tagged or not, it still needs to be RCD-protected in certain circumstances, such as outside in damp conditions. The two things are complementary, not interchangeable. You can have a fully tested and tagged item plugged into a socket with no RCD protection at all, and you can have an RCD protecting an appliance that has never been tested. Testing and tagging is not mandatory; what is legally required is that equipment is electrically safe and maintained in a safe condition. To understand the wider process, our explainer on what test and tag actually involves covers it.
The three types of RCD you will see in New Zealand
RCDs come in three broad forms, and they differ in where they sit in the circuit and who is allowed to install them.
| Type | Where it sits | Who installs it |
|---|---|---|
| Switchboard RCD | In the switchboard, protecting a whole circuit or a group of circuits | A licensed electrician or electrical inspector |
| Socket-outlet RCD | Built into a power point, protecting whatever is plugged into it | A licensed electrician |
| Portable RCD | Plugs into an existing outlet, protecting the equipment downstream | No electrician needed — sold at hardware and electrical suppliers |
For many workplaces the portable RCD is the practical option, because it can be moved to wherever the risk is and needs no fixed installation. It is also the type most commonly used on building sites and for outdoor work.
How an RCD is tested
There are two very different kinds of RCD test, and it is worth keeping them separate in your head.
The daily test button check
Every RCD has a test button. Pressing it deliberately creates a small imbalance, and a functioning RCD will trip. This is a simple, safe check that any worker can carry out, and it is the single most useful habit you can build. The guidance for construction sites is that RCDs should be test-button tested every day, including the RCDs built into portable socket-outlet assemblies (PSOAs). If the RCD does not trip when the button is pressed, stop using it and have it checked.
Instrument testing
The test button tells you the RCD trips; it does not tell you how fast, or at what current. That is what instrument testing measures. A technician uses a purpose-built RCD tester to confirm the device trips within its rated time and current limits. This catches slow or marginal devices that would still pass a casual button press. Instrument testing is the kind of work described on our RCD testing service page, and it is normally combined with a wider test and tag visit.
How often should an RCD be tested?
There is no single interval written into law for RCDs in general. How often testing is appropriate depends on the equipment, the environment and the risk profile of the work — the same principle that governs testing and tagging frequency. Our article on how often electrical equipment should be tested sets out how that risk decision is made.
What the guidance does establish for construction work is more specific, because the environment is harsher:
- Every day — a test-button check of every RCD, including those on PSOAs.
- At least every three months — PSOAs tested by an electrician.
- At least every six months — a periodic assessment of the builder's temporary (the temporary supply switchboard), with a Certificate of Verification issued by a qualified electrical worker.
Those figures come from construction guidance. They are a sensible starting frame for other high-risk environments, but they are not a blanket legal interval for every workplace in the country.
RCDs and construction sites
On a small construction site, the expectation is that all electrical equipment is RCD-protected, and that includes domestic appliances such as kettles and microwaves brought onto site. Harsh conditions damage leads and tools and shorten their working life, so the margin for error is smaller than in an office.
A builder's temporary, which supplies power before a permanent connection exists, must be installed and maintained by a licensed electrical worker and must comply with AS/NZS 3012 to be considered electrically safe. Before it is used it needs a Certificate of Compliance, a Record of Inspection and an Electrical Safety Certificate. Our guide to test and tag on construction sites goes through the site paperwork in more detail.
Common mistakes to avoid
- Assuming test and tag covers RCD protection. They are separate controls, and one does not substitute for the other.
- Never pressing the test button. An RCD that has quietly failed gives no warning until the moment it is needed.
- Using a domestic multi-board as a PSOA. They are not the same thing. A compliant PSOA is marked to AS/NZS 3012 and is tested by an electrician on a regular cycle.
- Ignoring a device that trips repeatedly. Frequent tripping is a signal to investigate, not to reset and carry on.
- Keeping no record of what was checked. Without a record you cannot show what was done and when, which matters both for safety and for your own peace of mind.
Daily checks and visual inspection
Instrument testing and button checks are only part of the picture. WorkSafe NZ advises that, whether you decide to test and tag or not, it is important to regularly look at cords, plugs and tools to see if they are damaged. A damaged lead can look perfectly normal from a distance, so a quick visual check matters more than it sounds. It takes seconds and can catch a cracked plug, a taped-over cord or a tool with a damaged casing before it becomes a hazard.
A workable daily routine for a site or workshop looks like this: check for new workers, new risks, new equipment and changes to the layout; press the test button on each RCD you depend on; give the cords and plugs in use a quick look; and set aside anything that appears damaged. Written down, that routine also becomes a record that you are actively managing the risk.
When to bring in a professional
Anyone can press a test button, and daily button checks should be part of your routine. Instrument testing, repairs and anything involving fixed wiring are different. Only a registered electrical worker may repair a fault, and switchboard and socket-outlet RCDs must be installed by a licensed electrician or electrical inspector.
WorkSafe NZ does not require testing and tagging, and it does not require you to use a contractor — a business can buy the equipment and train a worker instead. What it does say is that whoever carries out the work must have the knowledge and skills to do it correctly and to understand the relevant Standards. If you would rather not carry that in-house, you can request a quote for RCD and electrical testing.
Building RCD testing into your safety routine
The strongest setup layers its controls: RCD protection appropriate to the environment, a daily test-button habit, periodic instrument testing on a schedule that matches your risk, routine visual checks of cords and plugs, and a written record of it all. That combination is practical and defensible, and it does not rely on any claim that testing is compulsory.
If you would like to see how RCD testing fits alongside the rest of your programme, get in touch to talk through your site and the equipment in use.
