Intrinsically Safe blue cable

[Lagentium]

Evening all, could someone enlighten me please as to what the function is of the small screen wire in an IS cable and where it should be terminated please. Bit short, but that's all really!!

Cheers.

 

[Deleted member 26818]

Not sure what an IS cable is, but usually a screen is there to stop interference from electromatic and another sources.
Usually the screen is connected to Earth at one end of the cable, usually at the origin or source.

 

[westward10]

Evening all, could someone enlighten me please as to what the function is of the small screen wire in an IS cable and where it should be terminated please. Bit short, but that's all really!!

Cheers.

It is possibly the earth which is used to earth the IS equipment and is separated from the general electrical earth and is not there to carry fault current. Why do you ask because if those cables/circuits are incorrectly installed you could, depending on the environment asking be for serious trouble.

 

[Lagentium]

I'm asking because I have to connect some at work Westy, I have my CompEx, but I've never worked on IS really, in fact, at all. I'm just being cautious really, I will see when I get the panel drawings tomorrow, it's been on my mind since I glanced down at the cable after I cut it today!!

 

[westward10]

Got you, I am no expert with these matters but I suspect if it is damaged it may require replacement.

 

[Bobster]

Depends on what type of IS install it is. An what type of barriers are being used.

It is most likely a drain wire for a shied, but where it connects its depended on how it is separated, either galvanic, optically etc...

 

[KennyKen]

Depends what instrincally safe instrument/component your terminating.

 

[Lagentium]

The equipment is mainly pressure sensors and level switches. The pressure sensor is a Siemens product with a solenoid cap on basically. The drawings show me that the field end is made safe within the equipment rather than connected to earth and the other end in the panel goes to the IS earth, mounted below the barriers. Sound right? Don't ------- me Ken!

 

[Bobster]

Sounds right, but confirm it with the designer.

 

[timbobelfast]

Normal 'IS' concept is that the screen floats in the field and is connected to earth in the safe areaat the barrier.

Are Zener barriers being used?

 

[Lagentium]

That tallies with the drawing Timbo, I shall look at the barriers tomorrow and report back, if I'm not too upset at missing the rugby!

 

[Cbuk2k2]

90% of instrument cables are supplied in 1 pair, 2 pair etc format because most instruments are loop powered (the 24v going to the instrument is also used to send the measured pressure etc). Each pair normally has a screen wire for noise immunity. If your running your instrument cables next to a power cable for instance. The screen should form a circuit all the way from the marshalling cabinet in the equipment room to the field. The drawing will tell you what to do in the field but in 99% of instances you tie it back with a bit of heat shrink or earth sleving over it. At the marshalling cabinet this should be connected to earth. If your using zener barriers (old stuff) you often find a dedicated IS earth in the panel which this should be tied to. With more modern galvanic isolators these are often tied to a clean earth.

 

[LeeH]

It should be terminated EXACTLY as the drawing says.

No matter what anybody on here says or you think.

That's is the golden rule of the concept.

 

[Cbuk2k2]

I'd say it was pretty poor practice to blindly folow any drawing.

 

[timbobelfast]

It should be terminated EXACTLY as the drawing says.

No matter what anybody on here says or you think.

That's is the golden rule of the concept.

Not if the drawing is incorrect and you know it!

Zenner barrier earths common with 'plant' earth instead of the 'clean' IS earth!

As I have discovered after an instrument install was commissioned!

 

[LeeH]

If the drawing is incorrect then the drawing gets re issued.

You do not deviate from the drawing.

It's compex 101