Loose jumper - Why?

[AARUSS]

Hi
Can somebody explain to me exactly what a loose jumper is in relation to stop cocks not woolly sweaters and why it would be specified. I have been pricing a mechanical specification for one of my clients and it specifies the stop cocks but states that the cold water main stop cocks should have a loose jumper. I' am pretty sure it will not affect the price but it has got my curiosity. I have asked somebody already but their answer made no sense. From what I could gather it is the gate in the stop cock and if not loose it rides up and down fixed to the tap, they told me that it is fixed so that if used on a LP system it will still open and close but that makes no sense that it would be specified as loose for the mains cold water, I see why you wouldn't use it but why would you. Perhaps I'am missing something please be gentle with me if I'am being particularly stupid.

 

[sooey]

It will stop any backflow into the cold main.

 

[matt1e]

Probably to do with the non-return function of them

Matt

 

[matt1e]

I'm getting too slow at this

 

[Charnwood]

Tap jumpers:

The spindle is allowed to move up and down in the tap headworks;
up = open, held open by incoming pressure
down = closed, pushed down by back-flow of water