High voltage fuses protect the power distribution system from short circuits, overload and catastrophic faults. They are used extensively on transformers, switchgear and at substations, and even, albeit infrequently, on industrial power distribution systems. However perfect, in every sense of the word, the high voltage fuse may be, it will be of no use if not selected for the job, installed, and then maintained. Here we discuss the practical means of high voltage fuse inspection that lie in the hands of the engineer in the field. This article applies to working engineers having high voltage fuse links, current limiting fuses, and fuse cutouts at 11kV, 33kV, and 36kV respectively.
The first thing and apparently the most important thing to us engineer’s, in regard to high voltage fuse, it is its vital character generally, for unlike the low voltage variety which we are personally familiar, in taking over the job we are arriving in a situation endangering to central plant which could arise mutatis mutandis, upon said failure of the fuse, the possible results being:
Transformer damage;
Switchgear fuse failure;
Prolonged power cuts;
Fire or arc flash;
Loss of revenue while plant replaced. Making routine inspection seeking these, so long it gives us assurances of the fault condition, the fuse exactly operates rather on the intended and predictable basis. Sometimes, of course, it fails, but it is not the fuses fault but rather of age, environmental stress and/or mechanical damage.Common Types of High Voltage Fuses to Inspect
It goes without saying that before you attempt to inspect a fuse, you’d better know what you are looking at so as to be an engineer can tell which of these varieties he is examining at any given time:
· High voltage current limiting fuses;
· High voltage fuse links;
· High voltages fuse cutouts;
· Power fuses for transformers;
· Indoor high voltage fuses;
· Outdoor high voltage fuses.
There are points to consider unique to each kind, but most fuses are simple and each of these has certain basic points to consider, and which we’ll outline here.
Visual Inspection: The First Step and the Most Important One
Key Items to Check
Fuse Body Condition Without taking the lid off, you should detect if the fuse bodyceramic, for exampleis cracked, discolored, deformed, or burned. A clean and intact ceramic fuse body should display no signs of any kind of damage, because that indicates there may be deterioration within, too. Replace a damaged fuse.
End Caps and Terminals Look for signs of corrosion, looseness, or overheating. High contact resistance of terminals may occur for several reasons and is a common reason, among others, for causing a fuse to operate early.Fuse Cutout Alignment (Outdoor Systems)
If you are inspecting high voltage fuse cutouts in substations or overhead lines, make sure that the fuse tube is properly seated and aligned in its cutout or you may be courting failure of that cutout to do it’s work of interrupting current.
Signs of Partial Operation
While sparkplug-type current limiting fuses may give no indication of having operated other than the fact they are burnt out, some give subtle indications of partial operation. Look very carefully at any fuse that appears abnormal.
Mechanical Inspection and Installation Check
Sometimes, the mechanics don’t get a second thought. But they too can adversely affect the performance of the fuse!
Check that the fuse is held snugly.
Check torque on mounting hardware.
No excessive vibration on the fuse due to nearby equipment.
Adequate contact pressure on the fuse from clip and holder.
Loose or misaligned mounting can lead to localized heating and to have an effect on interrupting ability.
Electrical Inspection and Testing
DO NOT attempt any such testing on Plant high voltage fuses unless you are qualified to do so! Special rules apply.
Continuity Testing
A simple continuity test should confirm that the fuse element is still there! But continuity will not guarantee correct performance of the fuse in the presence of a fault.Insulation Resistance Testing
Measure the insulation resistance between the fuse terminals and ground. You may find low values due to moisture condensation inside the case, dirt or pollution on the surface, and even in the insulation of the fuse!
Resistance Comparison
Investigating the resistance readings of other fuses, that are identical in all respects, in the same installation may prove of value in running down an abnormal showing or internal damage.
Environment
A high voltage fuse may be located in a most severe environment indeed. Examine closely fuses located in: Sea-growths High humidity Duck and pollution Severe temperaturesFuses sited in coast market, industrial environment or heat (from ovens) . . . take extra care of those, and shorten the inspection period, likewise.
Don’t Inspect Fuses Unless
You are going to; that is no visual inspection (and no electrical checks). Ignore slight discolouration and corrosion. Use fuses (and types) of wrong rating. Fiddle with mix-match system fusing – please don’t do this. Put the old fuseholder back with the new fuse link – the sheathing will hold heat for too long a time. Forget about keeping a log of, results etc. of the inspections; one advantage to a proper log is that ‘trends’ can be established, rather than waiting for periods thorough fix. Inspection and testing will depend on the system of course. Roughly (and I mean roughly), inspection too often, below. Substation fuses etc., and transformers – annually; Fuses subject to sea-spray etc. – annually, or every six months; Critical industrial fuses, fuses in critical areas and should be inspected during every scheduled shut down. Fuses with faulted equipment should be inspected even if the fuses did not operate. When to Change a Hi-Volt Fuse Did it operate, (or at least partially operated), there is visible damage, unacceptable terminal corrosion, insulation resistance of such a low value – and more than this, the fuse rating and the rather simple question of whether it ‘agrees’ or not with the nature of system faults. Beware EVER attempting to repair damaged high Volt fuses.
An important note to Power Engineer and the Emergency buyer
Screw up fuse purchasing, and you ruin all the rest of the very clever work put into fuse inspections. It is quite necessary you stop and read carefully about high voltage fuses – and their source. Who makes the fuses? International standards?11 and 33 (or 36) kV power; and how to do it right!
