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Author Topic: Banana plug mishap  (Read 4970 times)

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Banana plug mishap
« on: January 11, 2013, 03:34:06 PM »
I would like to share a mishap with some banana leads I had, with the forum; it was a surprise for me when I found out, and since it could be a potential safety hazard, I thought to make a post about it.

Some time ago I decided to get a pair of banana plugs to use with my breadboard's binding posts. I cut crocodile test leads in half and stick them on the other end, so to have an easy way to bite on battery terminals, battery packs and such.
I also ordered a pair of test leads with hooks to use with my DMM, for more convenient testing.
My meter is the METEX M-3850D, bought many years ago. I like it very much, I had no other trouble with it until I discovered something weird. A while ago I opened it up and took a good look inside, sort of teardown/curiosity thing and I found out that both the plastic sleeves on the input jacks (COM, V/Ohm) had a crack on them!

After a moment of reflection I realized what had happened. Instead of using the test leads with the hooks (which I had stored in a box at the time), I had used those DIY test leads with the banana plugs which were in front of me on the desk readily available! Big mistake!
As you can see from the photos below, the plastic sleeve on the input jack of the multimeter extends quite a bit over the metal. So as you insert your probes, the male banana plug first touches on the plastic slightly and then presses on the metal snugly. However, these specific banana plugs, they proved to be just a bit fatter and caused the plastic to expand beyond its tolerance and made it crack! If the plastic sleeve was not this tall, or if the banana plug was a little more bendy it would had made contact with the metal as I pushed it in and it would be OK.


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Re: Banana plug mishap
« Reply #1 on: January 11, 2013, 03:50:57 PM »
It is interesting to see how uniform the crack it is on both of the plastic sleeves!
(yeah I had found those tiny flakes of plastic floating inside afterwards)

Also, I would like to hear your opinions regarding safety, does this change my meter's safety considerably?

I should tell you at this point that I am not touching any high energy/power systems at all. The higher I go is test some filter cap on a power supply <400VDC (~350V the higher I have seen so far), and occasionally I test my mains power 230VAC, other than that I just do typical hobby electronics low voltage stuff.

IMHO I am OK, I should not worry about it, since there is already the double insulation from a) the plastic case of the meter (dark grey) and from b) the plastic protection sleeves on the male banana plugs of the test leads I plug in the multimeter. Those plastic sleeves on the input jacks are the third level of insulation.

Also in the specific meter the clearances around the input jacks are large enough not to raise any eyebrows, especially for my needs/usage of the meter. Here are a couple more pictures to assist with the discussion:

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Re: Banana plug mishap
« Reply #2 on: January 11, 2013, 05:25:58 PM »
There is many parameters in this equation which deserve an individual analysis.

The banana at the first picture looks to be an made in USA Pomona product, which is considered as high quality one.

The binding banana in the other pictures are the most cheapy Chinese version, which I personally hate.
My problem with those is that a poor manufacturing process could easily lead to the fact, that the center pin  get out of place, and the banana becomes unusable.

I could not possibly imagine up to date, that a poor manufacturing process could also lead in to an over sized diameter.
But this detail it can be examined  ( or better said ) been measured by a caliper.

Regarding the inner insulating sleeves (multimeter inputs), they should withstand some pressure but if the material is hard plastic (Old days design) it could get damaged especially in cold temperatures which affect those materials negatively in the regard of hardness.

BRYMEN from Taiwan, offers such spare parts for their meters, and it looks like an Oasis in my eyes, due the fact that I am not aware of any other manufacturer, which has a similar offering today.
Regarding Safety, I would not worry, mostly because those are a single wire inputs, the damage in those inner sleeves, it is mostly an irritation for the mind in the first steps ( small inner cracks).

If I was in your shoes, I would try to add one or two rounds of support in those sleeves by using  clear sticky tape, the one which is commonly found in every office supplies shop in small tape rolls.
Metex and spare parts looks as dead end in our times.
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