fuses

[majuba]

Good day


A number of fuse failures have being occuring on our plant. I have checked the tightness of the wires/connections. They seem to be in good condition. Properly rated fuses are being used. I have a concern that most of the failures are occuring on the supply side. What could I look at to resolve this problem. The fuses are housed in fuse holders.

Thanks in advance.

 

[Skogsgurra]

majuba,

Are you talking about fuses for machinery or fuses for lighting? Or both?

Are the fuses hot in normal operation? Or are they just normally warm?

Is your voltage normal and stable?

Has any new equipment been installed lately? (Frequency inverters and thyristor controllers generate harmonics that tend to be shorted out by power factor compensation capacitors and thus overload fuses for flourescent lights with capacitors attached in parallel).

If you have a current transducer and a scope available then check the current waveforms in the fuses that fail. If you have waveforms that deviate a lot from a sine wave then have to find the cause for it and find alternative solutions.

Please keep us informed how you solve this problem.

 

[majuba]

the fuses are used not for lighting. the problem in particular occurs on the circuitory for an AC lubrication oil pump (for the turbines).

the fuses are warm in normal operation. voltage is a 380V supply.

no equipment has been installed

I am currently monitoring the problem

 

[rodar]

Intermittant fuse failures for circuits driving
motors is frequently a mechanical problem that
temporarily overloads the motor.
Connect a digital O Scope with a current clamp to the
circuit and set the trigger for one shot and
the threshold 30% higher than the fuse rating and
you might capture a waveform indicating a
temporary mechanical problem.
Use low bandwidth current probe to avoid triggering
the scope with noise.
Rodar

 

[CHDean]

You stated that the failures are occuring on the supply side. Are we to assume that you are melting the fuseholders or fuse body itself? If this is the case there are a couple of things to look at. 1.) You may have lost the spring tension in the fuseholders causing high resistance connection. 2.) If there is a switch directly upstream of the fuses it may have a pole with a poor connection generating heat on the load side.

 

[PUMPDESIGNER]

CHDean is very correct, fuseholder blades and springs weaken over time causing the snowball effect:
Added resistance, lower voltage, higher current,
Then more heat from higher current, lower voltage, higher current, ad infinitum.

Check motor contactor also, if weak then current increases.

Check fuse size. NEC range is 125% - 250%, but for inductive loads such as pump motors you do not want to be at the lower end of that range but rather try to be at 150% minimum, preferably 175% or higher if motor is started frequently with short time between starts. OK, so then some will automatically ask why increase fuse size, is that not dangerous? If original fuse was sized too close and was always on the very edge, then slight aging problems in electrical supply and equipment could push you over the edge making it appear that some problem developed. If anyone remembers, ask them if this fuse ever required replacement. If the fuse was always on the edge I bet they had to replace the fuse once in a while but not too often.

Did the operation of the pump change? Before pump may have been started infrequently but now starts more often. Did flow rate of pump increase? Was grade of oil changes to a heavier type?





PUMPDESIGNER

 

[jbartos]

Suggestion: The input side of fuses may be subject to voltage surges, wave reflections, and current spikes that may damage the fuse holder on the input side. The power supply quality analysis might be a good point to start with.

 

[Bung]

Can you be a bit more specific about the "failures"? Are you getting unexplained melting of the elements, or is there some sort of damage to the fuse body or holder? I presume you ae referring to 'cartridge' fuses (sand filled, ceramic body, fault current limiting). What type of connection is used - DIN blade fuses, bolted or what? What is the fuse environment? - high temperatures mean you will have to derate the fuse, and the materials used for the contacts limit the maximum temperature rises allowed.

Bung
Life is non-linear...