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  • Author Author: cookmesomekai
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How to kill a battery tester

cookmesomekai

A very popular HIOKI instrument is the BT3554 battery tester. It's a portable instrument designed primarily for maintenance purposes with voltage ranges up to 60V.

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Of course the BT3554 won't blow up when you accidentially apply 61V to it instead of the maximum 60V. In fact there are not many service cases for the BT3554 at all. However, about 9 out of 10 BT3554 that arrive in HIOKI's service department in Frankfurt, Germany are identified to have suffered some sort of over-voltage. The majority of those cases could have been prevented simply by replacing the original fuse in the BT3554 with the exact same type of fuse once it blew - which typically happens after the BT3554 has been subjected to some sort of overvoltage.

 

So the easiest way to kill a BT3554 battery tester is by replacing a blown fuse with something else and then have another overvoltage situation occur. To be fair, replacing a fuse with a different type or value to the one specified would probably damage all kind of T&M instruments in an overvoltage situation and not only this specific battery tester.

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The correct fuse for the BT3554 is a fast acting 630mA ceramic fuse (indicated by the "H" next to the capacity). Ceramic fuses have much higher breaking capacities than normal glass fuses. If a fuse needs replacing in the field to be able to continue with a job then it is typically more likely that there is a glass fuse flying around somewhere rather than a ceramic one (there is a spare fuse coming with every BT3554, by the way). However, replacing a ceramic fuse with a glass fuse (and even worse with any available value) makes it more likely that the instrument gets damaged when subjected to an overvoltage in the future. That might happen months later when it has been long forgotten that the glass fuse found in the van at the time was only to be used as a quick fix with the firm plan of ordering the right one once back at the base. But as you can see the required fuse in the BT3554 is nothing special but a standard ceramic fuse which is available from the same manufacturer (Littlefuse) at Farnell (SKU #1354537). Get some of them, put them in the BT3554 box and you can significantly reduce the risk of a repair case.

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  • in reply to colporteur

    There's plenty to worry about with fuses if you delve into the subject.

     

    For example - the fusing characteristic comes in a number of different ratings - the more common is F for "fast blow" and T for "time delay" or "slow blow". But some instruments are very sensitive (e.g. power analysers) and demand "FF" rated fuses which are super-fast. Swapping in a regular F-rated fuse risks causing permanent damage if an overload is to occur.

     

    There's the voltage on the fuse - most mains fuses are rated 250V or something like that, but many pieces of equipment in the T&M world require higher rated fuses, often up to 1000V due to their safety rating. Replacing one of these with a 250V rated fuse is not safe and would violate any safety category rating on the device.

     

    The issue alluded to here is actually related to the breaking capacity of a fuse - glass fuses are actually quite horrible, some have just a measily 32A stated breaking capacity. This is because as the fuse wire vaporises inside the chamber, it can form a conductive plasma which takes a while to settle down and break the arc. If the source has a larger current capacity, the result is often a fuse explosion (and lots of messy glass bits to deal with). Ceramic style fuses are often packed with silica/sand for arc quenching, some feature tens of kilo-amperes of breaking capacity, thus safely opening up the circuit in case of an overload.

     

    A number of reputable DMMs use specially sized fuses which are not used in other consumer equipment so that you can't just swap in any-old 3AG or M205 style fuse. This ensures that only DMM type HRC fuses are used, although you could still be a victim of "counterfeit" fuses in some cases and of choosing the wrong type as there are sometimes subtle differences in specifications.

     

    - Gough