A fuse is a calibrated weak link in a circuit that will predictably and reliably melt when a predetermined magnitude of current is reached for a designated duration. When the fuse element melts, the circuit is interrupted and the current will cease to flow. There is an inverse-time relationship between the size of the current and the time it takes to blow: the higher the current, the faster the fuse will open.

Fuses are sized according to their rated current and voltage. The current rating of a fuse should be between 0 and 30% higher than the continuous operating current in the circuit, depending on the type of fuse and the standard to which it complies.

Fuses used in North America typically comply with Underwriters Laboratories (UL) and/or Canadian Standards Association (CSA) standard 248-14 for low-voltage fuses (under 600V), while those used in Europe comply with International Electrotechnical Commission (IEC) standard 60127-2. UL and CSA standards are harmonized but they differ from IEC standards.

Table 9.1 shows the allowable continuous operating current for various types of fuses at 23°C.

If the rated current of a fuse is undersized, then it is subject to nuisance tripping due to fluctuations and spikes in the line voltage. If it’s oversized, it can be a potential fire hazard or a hazard to personnel by allowing too much current to flow.

When you are replacing a fuse, it is important to use the same fuse type, since UL and CSA ratings are different from IEC ratings. For a 250V fuse, for example, a 1.4-amp UL/ CSA fuse is approximately the same as a 1-amp IEC-rated fuse.

Therefore, if a fuse manufactured to UL standards is replaced with a fuse manufactured to IEC standards, then the circuit will no longer be protected properly. And it goes without saying that it’s never a good idea, regardless of the circumstances, to bypass a fuse with a chewing gum wrapper or any other conductive material.

It is also very important that the fuse is rated at or higher than the circuit voltage, or there is a risk of arcing across the open fuse terminals, thus bypassing the overcurrent protection. Furthermore, a fuse with the wrong voltage rating will work just fine until the fuse link blows and an arc is generated across the terminals.

Therefore, it is extremely important to pay close attention to the current and voltage ratings of replacement fuses. A properly rated fuse is designed to withstand the open circuit voltage for 30 seconds after the fuse blows or to have an interrupt resistance of at least 1 k ohms.

There are several different types of fuses that are classified according to how quickly or how slowly they will open in a fault or overcurrent situation. A fast-acting normal fuse will blow more quickly than a timedelayed or time-lag fuse will; although, you can overload a fast-acting normal fuse by 50% and it might still take several minutes to blow.

Time-delayed fuses are used in situations where the inrush current is high and the steady-state operating current is lower. Examples include discharge lamps, motors, transformers, and other highly capacitive or inductive loads. In the power distribution systems that we typically deal
with, we come across fuses on a regular basis in certain applications.

The secondary sides of feeder transformers, for example, are sometimes fused. So are the inputs to some large permanently installed dimmer racks. They sometimes use large bolt-in fuses filled with quartz sand to quench the arc and absorb the heat generated by stopping large currents. This type of fuse is occasionally called by the trade name Amp-Trap.

Of course, many luminaires are fused with miniature fuses, and some connectors in the UK have built in fuses. But the BS-546:1950 15 A connector is commonly used in theatres in the UK precisely because it doesn’t have a fuse.

And since every circuit is already protected by a fuse or circuit breaker in the dimmer, a second fuse in the connector is redundant and unnecessary. In fact, a second fuse in the same circuit makes it more difficult and time consuming to troubleshoot the circuit.

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