How much current can the following fuse withstand?

Thread Starter

szabikka

Joined Sep 3, 2014
84
Hello guys!

I need your help with identifying the current handling capabilities of a fuse. One of our lab equipments blew its glass fuse (two times, it also blew the spare one) which have the following marking: T200L250V. I know that the letter T stands for a slow acting fuse, the letter L stands for low breakage and 250V is the voltage rating, however I'm stunned by the 200 marking after the letter T. I don't think it's a 200 ampere fuse, given that it's fed from a 25 ampere wall fuse which didn't go off. The fuse has a somewhat thick filament in it, compared to a 2,5 amp one (which I have at hand), and it looks serrated on the surface. Does anyone have any idea about this little guy?

Thank you for your answeres in advance!
 

MisterBill2

Joined Jan 23, 2018
3,852
Before continuing with the fuse replacement you do need to locate the cause of such a failure, and repair whatever the problem is. Failure in that manner tends to indicate a direct short circuit which should not be that difficult to locate.
Fuses seldom fail for no reason, keep that in mind.
 

Thread Starter

szabikka

Joined Sep 3, 2014
84
Before continuing with the fuse replacement you do need to locate the cause of such a failure, and repair whatever the problem is. Failure in that manner tends to indicate a direct short circuit which should not be that difficult to locate.
Fuses seldom fail for no reason, keep that in mind.
Yes, I know that. I have already realized that the connector used to carry the mains voltage to the two circuit boards can be plugged in in reverse, and that caused a short. I was stupid not noting which way it faced when I unplugged it.
 

MisterBill2

Joined Jan 23, 2018
3,852
Yes, I know that. I have already realized that the connector used to carry the mains voltage to the two circuit boards can be plugged in in reverse, and that caused a short. I was stupid not noting which way it faced when I unplugged it.
ERRORS HAPPEN, good design tries to reduce the probability of errors and also limit the damage that they can do. The good news is that you were able to find and fix the failure. So the diagnostic task is completed. If no damage is done then just another fuse of that same 200 milliamp and 250 volt rating should work. It will need to be a time-lag fuse as well.
But that is a very low rating and I am wondering if it was the correct size. Is there any nameplate on the device that states the power rating? Of course, since I have no idea as to what the device is, that may be a completely reasonable rating. Just attempting to address all the possibilities.
 

Thread Starter

szabikka

Joined Sep 3, 2014
84
ERRORS HAPPEN, good design tries to reduce the probability of errors and also limit the damage that they can do. The good news is that you were able to find and fix the failure. So the diagnostic task is completed. If no damage is done then just another fuse of that same 200 milliamp and 250 volt rating should work. It will need to be a time-lag fuse as well.
But that is a very low rating and I am wondering if it was the correct size. Is there any nameplate on the device that states the power rating? Of course, since I have no idea as to what the device is, that may be a completely reasonable rating. Just attempting to address all the possibilities.
Thank you for your elaborate answer MisterBill2. The instrument itself is a rotary shaker in a chemistry lab. The 200 mA fuse is on the high voltage side of the transformer and it's the same for all of the rotary shakers we have. They were made by the same company but some of them are analog only, while some of them contain digital circuitry (rotary encoders, LCD dispalys, etc). The one I fried was from a digital one. I added two capacitors to the rotary encoder because they were not debounced in the factory and if you moved the encoder one notch it jumped to the highest speed and our stuff spilled all over the place. Now it works perfectly, but I had to add a new fuse :) .
 

MisterBill2

Joined Jan 23, 2018
3,852
Very interesting indeed. Are the rotary encoders the mechanical contact type or are they the optical variety. I have had some interesting times dealing with encoder signals over the years. Only some of them produce a good square wave, some produce a sine wave. Thus reading all of the specifications prior to ordering them is rather important.
 

Thread Starter

szabikka

Joined Sep 3, 2014
84
Very interesting indeed. Are the rotary encoders the mechanical contact type or are they the optical variety. I have had some interesting times dealing with encoder signals over the years. Only some of them produce a good square wave, some produce a sine wave. Thus reading all of the specifications prior to ordering them is rather important.
The encoders require relatively large force to turn and they make a definite clicking sound upon rotation, so I'd say they are of the mechanical contact variety. I didn't replace them, just put two 220 nF caps from their signal pins to ground and the signal is not bouncing anymore. If you turn it one click, the speed increases/decreases 5 RPM as it should. It appears to be a faulty series from the factory because we have two of the same type and both of them had this bouncing contact problem till I tinkered with them :).
 

Hymie

Joined Mar 30, 2018
708
230V mains with a current of 200mA gives a power rating of 46W.

Ideally there should be a marking close to the fuse indicating the rating, failing that a power rating or current rating marked on the exterior of the equipment.

Time delay fuses are normally used where there are short term current surges, e.g inductive loads (motors) or capacitive charging.

Bear in mind that a fuse can normally pass 1.6 times its current rating for a considerable time, once the current exceeds twice its rating it will normally blow within 30 seconds.
 

MisterBill2

Joined Jan 23, 2018
3,852
OK, and thanks for the explanation on the encoders. One alternative that may also work is a shot of contact cleaner, if you can get the kind that does not dissolve plastic. That is more commonly called "tuner cleaner", but televisions have not used that kind of tuners in perhaps 20 years. It may also be called "control cleaner" since it is still used to clean rotary potentiometers. But it seems that adding the caps solved the problem, and you did not suffer the paperwork burden that some organizations would place on making such a change. At Delphi Diesel it would have been 3 weeks to rush such a change through, if it ha been an emergency situation. That may be why we don't hear much about them these days.
 
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