The breakers in your house do not protect you from a shock
This is one of the most widely held misunderstandings about a home electrical system. The breakers in the consumer unit trip on overcurrent and short circuit, which protects the cable from overheating and catching fire. They do not protect people.
What protects people is a residual current device, a separate component working on a different principle. It compares the current flowing out with the current returning. If they do not match, some current is escaping by another route, and that route may be a person, so it disconnects within a fraction of a second.
A great many houses in Thailand have a breaker on every circuit and no residual current protection anywhere. That is adequate for the wiring and offers nothing at all to the person touching a water heater with a fault in it.
Thirty milliamps is the dividing line
Residual current devices come in several sensitivities, and the one used to protect people is 30 milliamps, which disconnects before the current can do lethal harm. Higher ratings, 100 or 300 milliamps, are there to prevent fires from leakage in the installation rather than to protect a person.
This matters when buying or when an electrician proposes something, because if you want protection for people and end up with a 100 milliamp device, you have a component that works exactly to its specification and does not do the job you thought you bought it for. Ask for the figure and look at it on the device.
The other distinction is form. Some devices do residual current detection only; others combine it with the breaker function in one unit. The combined type takes less space in the board and makes per-circuit protection much easier to arrange, which has a practical benefit described below.
One device for the whole house, or one per circuit
The cheapest arrangement is a single device on the incoming main, protecting every circuit. That sounds good and is genuinely good for safety, with a drawback that leads people to remove them.
The problem is that when it trips, it takes the whole house, and you have no idea which circuit caused it. Finding out means switching breakers off one at a time and retrying, which is miserable work at midnight with a torch. Worse, if the fault only appears with humidity, on rainy days say, you may never locate it at all.
What works better is separate protection on the risky circuits: the water heater, the sockets in bathrooms and kitchen, and outdoor sockets, possibly with another device on the main covering the remainder. Then a fault takes out one part of the house and tells you immediately where it is.
The water heater is where you must have one
If you fit protection in one place only, make it this circuit, because it combines every condition of an accident: water, a wet person barefoot on a wet floor, and an appliance drawing serious current.
Most modern water heaters have residual current protection built in, which is good, with two things to understand. It protects the appliance only, not the cable running to it or any other socket in the bathroom. And it can fail like any other component, and when it fails it usually shows no symptom at all, still supplying the heater perfectly normally.
So press the test button on the unit once a month. If the power cuts, it is working. If nothing happens, that is a dead device and you have been showering with nothing protecting you.
The test button nobody ever presses
Every residual current device has a test button, and most people never press it once in the life of the house, although the manufacturer intends it monthly.
The reason it matters is that this is a mechanism sitting idle for years without operating. Its internal contacts can stick with dust and humidity. Pressing the test button forces it to operate once, which both proves it still works and keeps the mechanism free.
It is simple. Open the consumer unit cover, find the button, usually yellow or blue and marked with a T, and press it once. The device should trip immediately, and you switch it back on. Under ten seconds in total. Warn the household first, and do not do it while something is mid-task, like a computer with unsaved work.
If it does not trip, treat that device as unusable and replace it. Do not conclude that you did not press hard enough.
It keeps tripping. Should I remove it?
No, and removing it is what some electricians suggest because it makes the problem disappear immediately. It does, and it also makes the real problem disappear from view.
A device that trips often is always telling you something. The commonest cause here is moisture accumulating in an appliance or in an outdoor junction box, which shows up in rainy weather and vanishes when the sun returns. The other is old appliances with degraded insulation, especially old refrigerators, washing machines and water pumps.
The way to find it is to unplug everything on the affected circuit and reset. If it holds, plug things back one at a time leaving ten minutes between each. Whichever causes the trip is the culprit. If it still trips with everything unplugged, the fault is in the cable or a joint in the wall, which needs an electrician.
One more cause, easily fixed, is several appliances that each leak a small amount naturally, such as electronics with interference filters. Individually trivial, together enough to reach the threshold. Splitting the load across more circuits solves it without reducing protection.
The earthing question that always follows
People immediately ask whether a residual current device works in a house with no earth. It does. It still detects current escaping through a person to ground and disconnects normally.
But an earth is much better, because when a fault appears in an earthed appliance the current flows to earth immediately and trips the device before anyone touches anything. In a house with no earth, the device trips at the moment someone makes contact, meaning you do get a shock, just one short enough not to be dangerous.
If you are renovating anyway, running earth conductors everywhere is worth doing at the same time, and far cheaper alongside other work. Same principle as wiring a smart home before the walls close.
Signs your house already has a leakage fault
Leakage does not begin with someone being thrown across the room. It begins with small symptoms people overlook and get used to.
- A faint tingle in the fingertips when touching an appliance, particularly with wet feet or standing on wet tiles.
- The tingle disappears with shoes on, which many people read as static. Static is a single snap and gone; leakage keeps tingling as long as you are touching it.
- A breaker or device tripping only on rainy days, or after the floor has been washed.
- An electricity bill that has crept up without explanation, less common but possible with continuous leakage.
If you have the first symptom, treat it as urgent. Do not work around it by putting shoes on. What makes a shock lethal is not the voltage but the path the current takes through the body, and the day your feet are wetter than usual is the day that path changes.
Cost and installation
Residual current devices are inexpensive relative to what they prevent. Parts for a house run from the low thousands to the low tens of thousands of baht depending on whether it is one device or several, plus labour.
This is not a job to do yourself, not because it is difficult but because working in the consumer unit means isolating the incoming supply, and a wiring error leaves a device that does not function while looking perfectly installed. That is worse than having none, because you will believe you are protected.
What to ask of the electrician: a demonstration after installation, both the test button and a real measurement, and a note on the invoice recording the rating fitted on which circuits. That document is useful both when selling the house and if you ever claim on insurance.
If you are buying a resale house, open the consumer unit cover on the viewing. A tidy board with proper residual current protection usually belongs to a house looked after in other respects. A board with tangled wiring and a mix of breaker brands and vintages tells you something about the repair history. What else to check at a viewing is in buying a resale house, structural risk and hidden costs, and the electrical system as a whole is in the home electrical system and your bill.
Frequently asked questions
What is the difference between a breaker and a residual current device?
A breaker trips on overcurrent or a short, protecting the cable from overheating. A residual current device compares outgoing with returning current and trips when some is escaping, which is what protects people.
What rating should I use?
Thirty milliamps for protection of people. Higher ratings such as 100 or 300 are for fire prevention from leakage rather than personal protection. Read the figure on the device itself.
Does it work in a house with no earth?
Yes. It still detects current escaping through a person and disconnects. An earth is much better, because it makes the device trip before anyone touches anything.
It trips constantly. Can I take it out?
No, because it is telling you leakage is genuinely present. Common causes are moisture in outdoor equipment and old appliances with degraded insulation. Find it by unplugging things one at a time.
How often should I press the test button?
Monthly, and it takes under ten seconds. If pressing it does not trip the device, that device is finished and needs replacing.
My water heater has one built in. Do I still need one in the board?
The built-in one protects the appliance only, not the cable to it or other sockets in the bathroom, and it can fail without any symptom. Fit one on the circuit as well and test both.
In short
- Breakers protect cable, not people. Residual current devices protect people.
- Thirty milliamps is the rating for personal protection. Higher ratings are fire protection.
- If you fit only one, fit it on the water heater circuit, which combines every accident condition.
- Press the test button monthly. If it does not trip, the device is dead.
- Frequent tripping is not a reason to remove it. It means leakage is real.
- A tingle in the fingertips with wet feet is urgent, and it is not static.
- Have the electrician demonstrate it and record the rating and circuits on the invoice.