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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