Fuse

What is a fuse?

A fuse is an electrical protective device with a fusible element that melts and permanently interrupts the circuit as soon as the current exceeds its rated value for too long or by too much. In doing so, the fuse protects cables, equipment and people against the consequences of overload and short circuit, such as overheating, damage and fire. In industrial automation fuses are everywhere: from the NH main fuse in the distribution board to the small glass fuse protecting a 24 V control circuit of a PLC.

A fuse is cheap, simple and extremely fast on a short circuit, but once it has operated it must always be replaced.

🧠 How does a fuse work?

The heart of a fuse is the fusible element. In small fuses it is a thin wire; in industrial fuses it is a silver or copper strip with narrow sections (notches), embedded in quartz sand inside a ceramic body.

1. Normal operation - the load current flows through the element. A fuse does have a small resistance: it warms up and dissipates power, which manufacturers state as the rated power dissipation. That heat therefore counts in the thermal calculation of a control cabinet.
2. Overload - with a moderate overcurrent the temperature rises slowly. A gG fuse of 16 A and above must not melt at 1.25 × In within the conventional time, and must melt at 1.6 × In within that time.
3. Short circuit - at a very high current all the notches melt almost simultaneously. This creates a series of small arcs that the quartz sand quenches. The fuse often clears within the first half-cycle of 50 Hz, before the short-circuit current reaches its peak.
4. After operation - the fuse is spent and must be replaced with one of the same type, rated current, rated voltage and breaking capacity.

That fast clearing is called current limitation. According to Siemens manufacturer data, a 224 A NH fuse limits a prospective short-circuit current of around 50 kA to a let-through peak of approximately 18 kA. As a result, busbars, cables and switchgear downstream have to withstand far less short-circuit energy.

🔍 What types of fuses are there?

Fuses are grouped by standard family and design. Low-voltage fuses fall under the IEC 60269 series; miniature fuses under IEC 60127.

Miniature fuses carry a speed code: FF (very fast acting), F (fast acting), M (medium time-lag), T (time-lag) and TT (long time-lag). They also carry a letter for breaking capacity: L for low (typical of glass tubes), H for high (ceramic, often sand-filled) and E for enhanced.

🏷️ What do gG, aM and the other utilisation categories mean?

On IEC 60269 fuses a two-letter code indicates what the fuse protects. The first letter is the breaking range: g covers the full range, from overload to short circuit; a covers short circuits only. The second letter is the protected object.

An aM fuse withstands the high starting current of a motor without melting. However, it is designed to break only high overcurrents: at 4 × In it must not operate within 60 seconds, and at 6.3 × In it must operate within 60 seconds. Lower overloads fall outside its breaking range, so an aM fuse does not protect the motor against overload. That is the job of the thermal protection in the motor starter, usually a thermal overload relay. Semiconductor fuses are often placed upstream of a variable frequency drive or soft starter, because ordinary fuses cannot protect thyristors, diodes and IGBTs fast enough. Whether they are mandatory is set out in the manufacturer’s installation instructions: many drives may also be connected with gG fuses, as long as you do not require protection of the semiconductors themselves.

🔄 What is the difference between a fuse and a circuit breaker?

A circuit breaker does the same job as a fuse, but can be reset after it trips.

Miniature circuit breakers to IEC 60898-1 have B, C and D curves, which trip magnetically at 3–5, 5–10 and 10–20 × In respectively. Fuses and circuit breakers are often combined: an NH fuse as back-up protection that limits the short-circuit current, with MCBs in the final circuits behind it.

🏭 Where are fuses used in an industrial installation?

• Main and sub-distribution - NH fuses or fuse switch-disconnectors protect supply cables to machinery, the MCC and HVAC installations.
• Motor circuits - aM or gG fuses as short-circuit protection combined with a contactor and thermal overload relay. Type 2 coordination under IEC 60947-4-1 requires the starter to cause no danger and remain fit for further use after a short circuit.
• Control circuits - cylindrical or miniature fuses for the control transformer, the 24 V power supply, sensors and actuators.
• Safety functions - manufacturers of a safety relay specify a maximum fuse rating that protects the output contacts against welding in the event of a short circuit.
• Hazardous areas - zener barriers for intrinsic safety contain an encapsulated fuse that protects the zener diodes; inside ATEX zones, fuses and holders must suit the chosen type of protection.

🔧 How do you select the right fuse?

A practical approach to selecting a gG or aM fuse:

1. Voltage - the maximum operating voltage of the supply (including tolerance) must not exceed the rated voltage of the fuse. Separate, lower values apply to DC, because direct current has no zero crossing.
2. Design current (Ib) - choose In ≥ Ib, applying the manufacturer’s derating factors for a high ambient temperature and for fuses mounted close together in the cabinet.
3. Cable protection - NEN 1010 (based on IEC 60364-4-43) requires Ib ≤ In ≤ Iz and I2 ≤ 1.45 × Iz. Because I2 for gG fuses of 16 A and above equals 1.6 × In, this works out at In ≤ roughly 0.9 × Iz.
4. Breaking capacity - this must exceed the prospective short-circuit current at the point where the fuse is installed. IEC 60204-1 and IEC 61439 require the short-circuit rating of the complete cabinet to be established.
5. Disconnection time - check that the fuse clears an earth fault fast enough, for example within 0.4 s for final circuits up to 32 A in a 230 V TN system. Long cables can limit the fault current so much that this fails.
6. Selectivity - for gG fuses in series, the standard ensures selectivity when the upstream fuse is at least 1.6 times larger, for example 160 A ahead of 100 A.
7. Motors - use the manufacturer’s coordination tables for the combination with contactor and overload relay.

Typical gG values according to IEC 60269:

One point to watch in 24 V DC control circuits: many switch-mode power supplies limit their current during a short circuit. A miniature fuse then receives too little current to melt, while the voltage for every other circuit collapses. Panel builders therefore often use electronic circuit protection per output, or a power supply with a temporary current reserve.

🔐 How do you work safely with fuses?

• Competence - NH fuses are intended for skilled or instructed persons. Under NEN 3140, the Dutch standard for operating electrical installations, you preferably replace them de-energised, using an NH fuse puller with a sleeve and suitable protective equipment.
• Fuse switch-disconnectors - these let you switch a circuit off safely under load and replace the fuses while de-energised.
• Isolation - removing a fuse is not automatically safe isolation. Follow the Lock-out Tag-out procedure, secure the isolation point and verify absence of voltage before you start, under a work permit where required. This is part of electrical safety in every plant.
• Never bridge or uprate - an oversized fuse or a bridged fuse lets the cable overheat and is a well-known cause of fire.
• Find the cause - a blown fuse is a symptom. Find the cause (short circuit, faulty motor, damaged cable) before fitting a new one.

📡 How do you monitor fuses from OT?

A blown fuse means downtime, often without a clear alarm. Many fuse holders and fuse switch-disconnectors therefore have an auxiliary signalling contact or electronic fuse monitoring that you wire to a digital input of the PLC. Electronic circuit protection modules for 24 V report the current per channel and the reason for a trip over a fieldbus or IO-Link. That data speeds up fault diagnosis and feeds predictive maintenance: a circuit that trips more and more often points to a wearing motor or a damaged cable. Also keep spare fuses of the correct type in stock; fitting the wrong replacement is a common mistake in practice.

❓ Frequently asked questions

What does gG on a fuse mean?

gG means that the fuse has a full breaking range (g) and is intended for general protection of cables and conductors (G). A gG fuse protects against both overload and short circuit. It is the most widely applied utilisation category in industrial distribution boards.

What is the difference between a fast-acting and a time-lag fuse?

A fast-acting fuse melts quickly even at a small overcurrent, whereas a time-lag (slow-blow) fuse tolerates a short peak without operating. Time-lag fuses suit power supplies, transformers and equipment with high inrush currents. Fast-acting fuses protect sensitive electronics.

Can I replace a fuse with a higher-rated one?

Replacing a fuse with one of a higher rated current is unsafe, because the cable is then no longer protected against overload. The fuse belongs to the cable cross-section and the installation, not just to the equipment. Always replace a fuse with the same type, rated current, rated voltage and breaking capacity.

Why does a fuse blow when a motor starts?

A fuse blows when a motor starts if its characteristic cannot withstand the starting current, which can be six to eight times the rated current with direct-on-line starting. An aM fuse, or a gG fuse chosen from the manufacturer’s coordination table, prevents this. Overload protection then comes from the thermal overload relay.

Is a fuse selective with a circuit breaker?

A fuse upstream of a circuit breaker is selective if the breaker clears every fault current before the fuse melts. That depends on the let-through energy (I²t) of both devices and can only be determined reliably with the manufacturer’s selectivity tables. The 1:1.6 rule of thumb applies only between gG fuses.

Why do fuses have a lower voltage rating on DC?

A fuse in a DC circuit has to quench the arc without the help of the zero crossing that alternating current has a hundred times per second. As a result, the same fuse is suitable for a lower voltage on DC than on AC. Dedicated fuses therefore exist for solar panels and battery systems, such as gPV fuses to IEC 60269-6.

📌 In summary

A fuse protects electrical installations against overload and short circuit by permanently interrupting the circuit with a fusible element. Choosing the right utilisation category (gG, aM, aR), breaking capacity and coordination with the cable and upstream protection gives you cheap, very fast and selective protection that can also be monitored from the PLC.