Porcelain Fuses, MCBs & RCDs Explained

What These Devices Look Like

Switchboard parts vary by brand and age, but these examples show the general idea. If your board is old, damaged, crowded or unlabelled, have a licensed electrician check it rather than trying to identify parts behind covers yourself.

Fuse exampleFuses come in many forms. Older homes may have porcelain rewireable fuse carriers, while mains and network equipment may use cartridge or HRC fuses.

MCBsOlder plug-in style MCBs and modern DIN-rail MCBs are resettable overcurrent devices. They protect cables from overloads and short circuits.

RCCBAn RCCB provides residual current protection only and must be paired with correctly rated overcurrent protection.

SRCD / RCD socket outletAn SRCD provides RCD protection at the socket outlet itself. This style was commonly used for local wet-area protection.

RCBOAn RCBO combines overload, short-circuit and residual current protection in one switchboard device.

PRCD / portable RCDPortable and inline RCDs add temporary leakage protection when working outside, using extension leads or plugging into older outlets.

Old, Common and Modern Board Layouts

These examples show why the device type matters. Two switchboards can both “work”, but the level of safety, fault separation and electric shock protection can be very different.

Old porcelain fuse boardOlder porcelain fuse boards can be unsafe if fuse carriers are damaged, incorrectly wired, poorly labelled or fitted with the wrong fuse wire. They also do not provide modern RCD shock protection.

Common RCCB and MCB layoutA common upgrade layout is one RCCB providing RCD protection to a group of circuits, with MCBs providing overcurrent protection for each circuit.

Modern RCBO layoutA modern board may use individual RCBOs for power and lighting circuits. This gives each circuit its own overcurrent and RCD protection, so one fault is less likely to turn off half the house.

What Are Porcelain Fuses?

Porcelain fuses are older protective devices commonly found in older New Zealand switchboards. They usually contain a small piece of fuse wire inside a removable ceramic carrier.

When too much current flows through the circuit, the fuse wire heats up and melts. This opens the circuit and helps protect the cable from overheating or being damaged by a short circuit.

The important point is that a porcelain fuse is mainly protecting the wiring, not the person. If someone receives an electric shock, the current through the body may be only a small fraction of the fuse rating. That may be enough to seriously injure a person, but not enough to blow the fuse.

Porcelain fuses can also be slow to operate in some fault conditions, especially where the fault current is not high enough to blow the fuse quickly. In older homes, it is also common to find fuse carriers that have been loaded with the wrong fuse wire size, oversized fuse wire, or poorly fitted wire after years of DIY repairs.

What Is an MCB?

An MCB is a miniature circuit breaker. It performs a similar overcurrent job to a fuse, but it is resettable. If it trips, the cause should be checked, then the switch can usually be reset without replacing fuse wire.

MCBs normally respond in two ways. A moderate overload heats a thermal element and trips the breaker after a delay. A large short circuit creates a strong magnetic effect and trips the breaker much faster.

MCBs are a big improvement for convenience and consistency, but they are still not designed as personal shock protection. A 20A MCB is watching for circuit current that is too high for the cable. It is not watching for a 30mA leakage current through a person.

What Is an RCD?

An RCD is a residual current device. RCD protection can come in several forms, including an RCCB, RCBO, SRCD or portable RCD, but the main principle is the same.

The RCD compares the current going out on the active conductor with the current returning on the neutral conductor.

Under normal conditions, the current going out and coming back should be balanced. If some current leaks away, for example through a damaged appliance, wet extension lead or a person to earth, the RCD detects the imbalance and trips.

Most domestic RCD protection is designed around 30mA residual current protection. The idea is to disconnect fast enough to reduce the risk of a serious or fatal electric shock.

The main difference between RCD devices is what else they do. An RCCB provides residual current protection only and needs MCBs or fuses for overcurrent protection. An RCBO combines both functions in one device. An SRCD provides RCD protection at the socket outlet, while a PRCD provides portable RCD protection for tools, leads and temporary use.

From RCD Power Points to Switchboard RCDs

In older homes, it was common to see RCD protection added only where the risk was obvious, such as bathrooms, laundries, wet areas, garages or outdoor power points. Sometimes this was done with an RCD power point or a local RCD unit near the area being protected.

The modern approach is different. From January 2003, new circuits originating at the switchboard and supplying lighting or socket outlets in domestic buildings in New Zealand required RCD protection. In practical terms, this moved RCD protection toward the switchboard, where one RCD or RCBO can protect the wiring and outlets downstream.

This does not mean every older home had to be completely rewired in January 2003. Existing installations can remain in service if they are safe, but new work, new circuits and switchboard upgrades are assessed against the rules that apply at the time of the work.

Later wiring rule updates have continued to strengthen how RCD protection is applied in residential installations. Today, it is normal to see RCDs or RCBOs installed at the switchboard, with ordinary-looking power points throughout the house protected from there.

RCD Power Points, RCCBs, RCBOs, SRCDs and PRCDs

RCD is the broad term homeowners usually hear, but RCD protection comes in several forms. The device may be built into the switchboard, built into a power point, combined with an MCB, or used as a portable inline safety device.

How Quickly Do Fuses, MCBs and RCDs Trip?

The answer depends on the size and type of fault. Protective devices do not all trip for the same reason.

A fuse or MCB responds to overcurrent. If the current is only slightly above the rating, it may take a long time to operate. If the current is a major short circuit, it should operate much faster.

An RCD responds to leakage current. It is looking for a small imbalance, commonly around 30mA in domestic shock protection, so it can trip even when the total circuit current is nowhere near enough to operate a fuse or MCB.

As a practical example, a 20A MCB may not trip quickly at a small overload. Standards for common MCBs allow a time-current curve where low-level overloads can take a long time, while high fault currents trip the magnetic mechanism almost instantly. Fuses behave the same general way: the more current flowing above the rating, the faster they operate.

Coarse protection vs close protection

Coarse and close protection are terms used for excess-current protection. They are about how accurately a fuse or circuit breaker disconnects when the current rises above the safe rating of the circuit.

Close protection means the device operates more accurately at lower overcurrents. Modern MCBs and correctly selected HRC fuses are examples of close protection because they have predictable operating characteristics and are designed to disconnect before a cable is exposed to prolonged overheating.

Coarse protection means the device is less precise and may allow a higher overcurrent to flow for longer before it operates. Rewireable porcelain fuses are a common example. Their performance depends heavily on the correct fuse wire size, wire material, condition of the carrier and how well the wire has been fitted.

For homeowners, the simple takeaway is that rewireable fuses are old technology. They can be cheap and simple, but they are easier to misuse, less predictable, and can allow a circuit to run overloaded for longer than a modern protective device would. This is one reason older porcelain fuse boards are often upgraded to MCB and RCD, or RCBO, protection.

That is why RCD protection is so important. A dangerous shock current through a person can be far below the current needed to blow a fuse or trip an MCB. An RCD is designed to detect that missing current and disconnect much faster.

Why RCDs Offer Far Greater Electric Shock Protection

Old fuses and MCBs are looking for too much current. That is excellent for protecting cables from overheating, but it is not enough for modern personal safety.

A person can receive a serious electric shock at currents far lower than the rating of a normal power circuit. A fuse or MCB may see that as normal load, because from its point of view the circuit is not overloaded.

An RCD looks for imbalance instead. If current leaves the circuit and does not return the normal way, the RCD trips. This is why RCD protection is one of the most important upgrades in an older home.

What Should You Check in an Older Home?

If your home has an older switchboard, porcelain fuses, unlabelled circuits or no obvious RCD test buttons, it is worth having the switchboard assessed.

• Does the switchboard still use porcelain rewireable fuses?
• Are there RCDs or RCBOs with test buttons?
• Are bathrooms, laundry, garage and outdoor outlets RCD protected?
• Are circuits clearly labelled?
• Are any fuse carriers cracked, loose, burnt or damaged?
• Has the home had new power points or circuits added over time?
• Would a switchboard upgrade improve safety and future capacity?

Frequently Asked Questions

Related Guides

These related guides may also help if you are planning electrical upgrades or checking an older home.

• Why Is My RCD Tripping?
• Why Are My Lights Flickering?
• Single Phase, Two Phase & Three Phase Power Explained
• Contact Chris Odlum Electrical