Banana Coupler 4mm for test leads extension = Not compatible for shrouded plugs

[Kiriakos GR]

At December 10, 2018  I wrote an article about a banana coupler 4mm for test leads extension.

I got one pair of unbranded Chinese production of banana coupler, and the number one detected problem this was matting compatibility with cable extensions loaded with shrouded (safety) banana plugs.

I thought back then that I were cheated, and that the machinery which made this banana coupler insulation, this was out of calibration.
And that this is the problem, for having an extreme tight fit with specific shrouded banana plugs.

Eight years later I did discovered the truth.
European and Chinese manufacturing sources, they are designing these banana coupler for use with regular non insulated 4mm male plugs.

Banana coupler external insulation sleeve, this does not translate to compatibility with shrouded safety banana.
Even XHK staubli,  these are made for spring loaded 4 mm plugs with rigid insulating sleeve. 
   
Translation please ....
If you do not have naked banana plugs,  and or banana plugs with spring loaded insulating sleeve, then you will not be able to use this type of banana coupler.

The matting issue, this has to do with tight fit, and also with planet earth physics.
example:
Imagine shrouded safety banana this be equal to car engine piston, when this is inserted in to female plug this causes air compression.
Shrouded part of safety banana, this acts as a perfect gasket, its totally cylindrical / round.

By matting together two pistons in to the same cylinder, this causes air pressure, and in combination with PVC to PVC insulator wall friction forces, we have the outcome,  that you need to use significant force so to perform this matting.

Naturally the mechanical devil here is sub millimeter differences.
If the male shrouded banana this has an wider shroud format this will work.
But there is no warranty that your branded test leads, they will succeed to get connected.

The one million dollars question:
Whom is going to manufacture an banana coupler for shrouded (safety) banana plugs ?

We use a banana coupler so to connect two male 4mm plugs.
From the other hand, using a male shrouded banana with female shrouded banana, this never seemed causing a problem at test leads line extension.