A fuse tube that drops by itself and causes an unplanned outage is the most annoying thing. Everything is running fine, then a little wind or vibration and the tube falls off. There are three usual reasons. First, the mounting angle is wrong. A dropout fuse should tilt 15 to 30 degrees. Too little angle, the tube won't drop; too much, the upper contact can't hold it and a small jolt makes it fall. Second, the fuse link isn't tight enough. When you install it, if you don't pull it taut, the moving contact doesn't put enough pressure on the stationary contact. Third, the release mechanism wears out over time and loses its holding force.
The fix is simple. Check the angle with a protractor and adjust it to 15–30 degrees. When installing a new fuse link, pull it tight – about 24.5N of tension feels right, don't leave it loose. If the mechanism is badly worn, replace the whole tube with a newer type that has a double‑latch design – that solves the problem for good.
Another common issue is one phase dropping while the other two stay closed – single‑phase dropout, which is dangerous. When a fault happens, only one fuse blows, but the other two phases keep feeding power. A three‑phase motor downstream will then run on two phases and burn out in minutes. The root cause is almost always that the three fuses were not replaced at the same time. They have different age, different brand, different material, so their time‑current curves don't match. When fault current hits, the weakest one goes first. Sometimes the rated currents look identical on paper but actually have tiny differences, and the smallest one blows first.
The rule is simple: if one phase blows, change all three phases at once. Use the same brand, same rating, and if possible the same batch. Saving a few dollars on two fuses isn't worth losing a motor.
Fuse links that keep blowing, but you can't find any fault on the line – that's another common headache. You swap the fuse, and a week later it's blown again. The line checks out fine. The cause is usually not a line fault. First, the fuse rating is too low. Normal load variations or the transformer inrush current are enough to blow it. Second, the contacts are corroded or loose – high contact resistance creates local heating that travels up the fuse link and melts it under normal current. Third, the fuse link itself is poor quality, or it was installed too tight or kinked, which reduces its cross‑section and creates a hot spot.
The fix: for transformers below 100kVA, choose a fuse link rated 2–3 times the full load current; for 100kVA and above, use 1.5–2 times. Clean the contact surfaces regularly and apply conductive grease. Buy from reputable manufacturers, and when installing, pull it snug but don't over‑stretch or bend it.
The most serious problem is a fuse tube that burns black or actually explodes. This usually happens during a short circuit. The arc doesn't extinguish and destroys the tube. There are a few reasons. First, the interrupting capacity is too low – the fuse's maximum fault current rating is lower than the available fault current at the installation point, so the arc won't go out. Second, the arc‑extinguishing tube (made of fiber or paper material) has absorbed moisture, so it doesn't produce enough gas to blow out the arc. Third, the tube is simply too old – after five years of service, the inner wall is carbonised and can't extinguish arcs effectively.
When selecting a fuse, calculate the maximum short‑circuit current at the installation point and make sure the fuse's interrupting rating is higher than that. In coastal or humid areas, use salt‑fog‑resistant types. Replace any tube that has been in service for more than five years – don't wait for it to fail.
A little routine maintenance prevents most of these failures. During every patrol, look for cracks in the insulator, deformation of the tube, signs of overheating on the contacts. Use a 2500V megger to measure insulation resistance – for 10kV systems it should be at least 1000MΩ. Once every six months to a year, do a proper service: clean dirt off the insulator and tube, remove oxidation from contacts and apply conductive grease, check spring pressure. If contacts are badly burned, smooth them with fine sandpaper and then grease them. In coastal or heavily polluted areas, switch to silver‑plated contacts – they resist oxidation much longer.
Dropout fuses aren't complicated. Most problems can be avoided with correct selection and regular maintenance. If you have a stubborn issue, send us photos of the installation. We can help figure out what's wrong and find a suitable replacement.