Difference between IS & NIS cable

Hi Craig, do you mean to say that IS cable can be used as NIS cable as well (if thr is no cable color terminology defined) . This leads to my understanding as . .wat ever barrier , isolatior comes (in case of 4 wire instruments like coriolis flow meter ) in on control room side so i need not worry abt it while selecting cables . Only thing is budget ..IS cable being more costly than NIS.. Plz give your opinion

Hi msn,

There is no such thing as IS or non-IS cable. What is important to know about IS loops is that you are limited by the capacitance, inductance and impedance of the loop - as you are limited by the energy you can allow into the hazardous area. The first thing you are going to select is your transmitter which will have x, y, z capacitance, inductance and impedance. The next important thing to know will be the length of cable to get to that transmitter. You will then make sure that your total capacitance, impedance and inductance fall within your countries regulations. This is how you will determine the cable size i.e. 0.5mm, 1mm, 1.5mm as each cable size has a different inductance, impedance and capacitance. You will then match your barrier or galvanic isolator to the instrument.

So for example here in SA I would order my cable for IS loops with a blue outer sheath, and for the non-IS loops the same spec cable but with a black outer sheath. Some cable manufacturers can offer a different insulation material i.e. polyethelene which helps if you have a requirement for low mutual capacitance. They might also change the average lay length to reduce cross talk.

So to summarise - there is no such thing as a specific cable for IS and a specific cable for no IS. The requirement being to limit the energy you can put into the area - and this is dependant on capacitance, impedance and inductance. This in turn will determine the cable size.

As stated earlier - the barrier/isolator must be matched to the transmitter. This is not only dependant on whether the loop is four-wire or two-wire, but also on the protection class employed i.e. Ex(ia), Ex(ib) or Ex(ic).

I hope this makes things a little clearer for you.

Regards,

Craig

Hi

The basic diff. wat I understand is that these are the hardwires that carry IS signals (Limited Power) which is achieved by using External device like Barriers.

The only special requirement for cables carrying IS circuits is that they must be specifically identified as such, and they must be segregated from other non-IS circuits.

For more information visit the site "http://www.control.com/thread/1026221506"

The link ll further clarify ur doubts.

TK

The inductance and capacitance of the cables is important, for these determine the energy-storing, and therefore spark-generating, capabilities of the circuit, barrier-fed or not.

In simple terms, the cables with the blue sheath, which in the UK usually will be to some part of BS5308, do not exceed the inductance and capacitance limits whatever their length, and therefore can be used without further consideration in EEx i circuits.

Job 38:11.

The same cannot be said for MICC cables, for example, as an explosion witnessed under demonstration conditions from a circuit using this cable fed via an EEx ia barrier device testified!

Hi, now I am quite clear abt IS cables but could you please explain bit more abt the example you have written of MICC cables ? Also why barriers used in case of MICC? Dont we use Galvanic Isolation ?

It's got nothing to do with galvanic isolation.

MICC cable falls outside the criteria for being low-energy-storing irrespective of length. If MICC is long enough downstream of it, a barrier-fed circuit will contain enough energy to cause an explosion in the hazardous area, regardless of the certification of the individual devices. [Been there, seen it, T-shirt now on eBay....]

So correct cable choice is important in EEX i circuits.

His msn,

Typically we can use two forms of protection to limit the energy in a loop with intrinsic safety. Either we use a barrier (zener) or we use a galvanic isolator.

With a barrier we would need an intrinsic safe earth. With a galvanic isolator there is no need for this.

Typically we would use galvanic isolators on a green fields project. While the isolator is more expensive than the barrier - it eliminates the requirement for inspections to the IS earth etc.

For more information on the differences between the two forms - try a search for MTL. You can then download the data sheets which will show you the differences in the circuits between the two.

Regards,

Craig

Hi Craig, how can isolator be used instead of barrier? Barrier restricts the energy in IS loops. Which can not be achieved by use of isolator and DCS cards may get damaged due to high energy.

Hi msn,

Galvanic isolators use a barrier in the ciruit to limit the energy. But they use galvanic isolation to seperate the hazardous area from the safe area.

Have a look at this site and download some of the datasheets then you will see the differences in the two circuits. www.mtl-inst.com

Regards,

Craig