A ceramic fuse is the older type of protection found in fuse boxes built before circuit breakers became standard. Each circuit has a porcelain carrier holding a thin length of fuse wire. When too much current flows, the wire melts and the circuit goes dead. It protects the cable from overheating, but it gives no protection against electric shock.
Plenty of Sydney homes built before the 1980s still run on them. They are also called rewireable fuses, because the wire is replaced after it melts. If you have pulled one out and wondered what you are looking at, here is how they work and why electricians keep recommending you replace them.
How does a ceramic fuse work?
A ceramic fuse works by letting a thin wire melt before the cable in your walls gets hot enough to be damaged. The fuse has two parts: a base fixed to the board, and a removable porcelain carrier that plugs into it. Inside the carrier, a short piece of tinned copper fuse wire runs between two screw terminals. Current for that circuit flows through the wire.
The wire is chosen to suit the circuit. A lighting circuit gets a lower rated wire than a power point circuit, and the rating is usually marked on the carrier or the board. When the current climbs past what that wire can carry, it heats up and melts, which breaks the circuit.
Some older boards use cartridge fuses instead, where the fuse element is sealed inside a small tube that is replaced as a unit. The principle is the same. Either way, these fuses sit on your side of the meter. They are not the same as the service fuse on the network side, which only a Level 2 electrician can replace.
Why does a ceramic fuse blow?
A ceramic fuse blows when a circuit draws more current than the fuse wire is rated for, either from an overload or from a fault. The common causes are:
- Overload. Too many appliances on one circuit, such as a heater, kettle and toaster all running from the same kitchen power points.
- A faulty appliance. A damaged cord or an internal fault in an appliance can draw a sudden surge of current.
- A wiring fault. Damaged insulation, a loose connection or a short circuit in the fixed wiring. On an older installation, this is more likely than it used to be.
If a fuse blows once when several heavy appliances are running, overload is the likely cause. If it blows again with little or nothing plugged in, the fault is in the wiring and needs an electrician to find it. The fault paths are the same ones covered in why a breaker trips with nothing turned on.
Ceramic fuse vs circuit breaker: what is the difference?
Both protect a circuit from overload, but a circuit breaker does it faster and more precisely, and it resets with a switch instead of new wire. Here is how they compare.
| Feature | Ceramic fuse | Circuit breaker |
|---|---|---|
| How it cuts power | Fuse wire melts | Internal switch trips open |
| After it operates | Carrier removed and rewired | Switch flicked back on |
| Response to overload | Slower and less precise | Faster, set to a calibrated rating |
| Can be defeated | Yes, with the wrong wire | Not without replacing the device |
| Shock protection | None | None on its own, needs an RCD or RCBO |
Neither a fuse nor a plain circuit breaker stops an electric shock. That job belongs to an RCD, the device most people call a safety switch, which trips when current leaks to earth. Modern boards pair every circuit with one, often as a combined RCBO. We explain the difference in RCBO vs RCD. Most ceramic fuse boards have no RCD at all.
Still running on ceramic fuses?
DM Electrical can inspect your fuse box, test the wiring behind it and tell you honestly what needs doing. Based in Brookvale, Level 1 and Level 2 ASP accredited, available 24/7 across the Northern Beaches and wider Sydney.
What is wrong with using the wrong fuse wire?
Fitting heavier fuse wire removes the protection the fuse exists to provide. The wire rating is matched to the cable it protects. A circuit wired for a lower current, fitted with heavier wire, can carry far more current than the cable is designed for. The fuse never blows, and the cable overheats inside the wall or ceiling instead.
Electricians find this often in old fuse boxes. When a fuse keeps blowing, someone fits a thicker wire, doubles it up, or in the worst cases bridges the carrier with a nail or foil. Each of these stops the fuse blowing by stopping it from protecting anything. Replacing fuse wire also means pulling a carrier out of a live board, often in the dark. If a fuse has blown, we recommend having an electrician rewire it and find out why it blew, rather than fitting whatever wire is in the drawer.
Are ceramic fuses still legal in NSW?
Yes, an existing fuse box that works and has not been altered can legally stay in place. There is no general requirement in NSW to remove a working ceramic fuse board. The current AS/NZS 3000 Wiring Rules apply to new work, so the question usually comes up when something changes.
Adding a circuit, installing an EV charger, a smart meter or air conditioning, or renovating a kitchen all involve work on the board. New and altered circuits need RCD protection, and a fuse board rarely has the space or the design to take it. That is usually the point where replacing the board makes more sense than working around it.
When should a ceramic fuse board be replaced?
Replace it when you are about to do electrical work anyway, or sooner if the board shows signs of damage or constant blowing. Warning signs worth acting on:
- Scorch marks, cracked carriers, or a hot or burning smell around the board.
- Fuses that blow regularly, or evidence that someone has fitted heavier wire.
- No safety switch anywhere on the board.
- A grey fibrous panel behind the fuses, which may be asbestos. See our guide to asbestos meter board replacement before anyone drills or cuts it.
A fuse board is also a clue about the wiring behind it. If the house still has its original cabling, it is worth checking whether it needs attention too, and our guide on whether your house needs rewiring covers what to look for. Our switchboard upgrade page explains what a replacement involves, and a residential electrical inspection will tell you where your installation stands.
Frequently asked questions
Are ceramic fuses illegal in NSW?
No. An existing fuse box that works and has not been altered can legally stay. The current Wiring Rules apply to new work, so adding or altering circuits usually means bringing that part of the installation up to date, which on a ceramic fuse board often means replacing the board.
Can I use a higher rated fuse wire?
No. The fuse wire rating is matched to the cable it protects. Heavier wire lets the cable carry more current than it was designed for, so it can overheat inside the wall without the fuse ever blowing.
Why does my ceramic fuse keep blowing?
Usually an overload or a fault on that circuit: too many appliances, a faulty appliance or damaged wiring. Unplug everything on the circuit before the fuse is rewired. If it blows again with nothing connected, the fault is in the fixed wiring and needs an electrician.
Do ceramic fuses protect against electric shock?
No. They respond to excessive current, not to current leaking to earth. Shock protection comes from an RCD, also called a safety switch, which most ceramic fuse boards do not have. That missing RCD is the main reason electricians recommend replacing them.
The bottom line
A ceramic fuse is a simple, old device that protects your cabling from overload and nothing more. It is not illegal, and a well kept one can keep working for years, but it has no shock protection, it is easy to defeat with the wrong wire, and it becomes a barrier the moment you want to add anything new. If your home still runs on one, get in touch and we will check the board and the wiring behind it.