Persistent Fuse Blowing

This brings back bad memories with a personally designed radio telemetry system that kept 'popping' fuses in the power supply on a regular basis. What was, however, most strange was this only happened in a town 300km from the office (rather an expensive round trip just to replace a fuse).

But then came the nasty follow-on. Technicians were starting to replace the fuse with a higher amperage which resulted in rectifiers being damaged, which in turn burned out mains transformers!

A quick phone call to the local electricity department revealed the cause. The town's main incoming transformer had tap changes exceeding 5%, and it was not uncommon for them to change by two slots in one go. This meant the mains input to the power supplies was often subjected to over 10% increases in one foul swoop, and the inrush through the fuses just a little too much to bear.

Although such stories are not uncommon, the reasons behind inadvertent, unexplained fuse failures (i.e. no apparent damage causing the fuse to blow) leaves many bewildered. Inrush current, that which exists when bringing a capacitor to full voltage from some rest state, is usually to blame. This inrush is just as intense when the input voltage jumps by a good proportion.

Although many designers opt for "slow blow" types, these too can easily fail under such inrush conditions. It must be remembered the current curve of a rectifier system is short but intense during the conduction phase. Slow-blow fuses have two ratings, the lower long term average current (the slow-blow rating) and the higher instantaneous rating - a rating that can easily be exceeded by the current curve of the rectifier. Such fuses are only suitable on the input to a mains transformer (where the impedances are much higher) rather than between the secondary and rectifier.

The cure for this is an 'inrush' resistor whose whole purpose in life is to minimise the maximum current the rectifier can draw. Cures are discussed later but it was felt this was a design issue and not really a power quality one. Recognising inrush issues like this is dealt with in the section Interpreting the Readings under "Flicker Induced Inrush".

Such persistent fuse blowing is not just limited to small circuitry or appliances. It could easily be found on buildings too. In one case the engineer became concerned when the main fuses for an office block blew as the power was restored after a maintenance shutdown. With each and every device that was switched on at the time power restored would have added to the inrush current and this was obviously too much for the fuses to tolerate.

Input Rectifier Diodes  >>

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