Capacitance of Intrinsically Safe Cables

Hi Engineers!
What is the correct way to calculate capacitance of IS cable? I have manual from GE that states: Ccable=Cwire-wire + Cwire-shield.
But MTL tech support tells me to use value that is bigger, as a rule it's a Cwire-shield. What is the right calculation?
Hi, PhilCorso
Actually I forget, that I've already posted my question, but it's still actual. There's not answer in standards, that you suggested.
The standards you refer to would not detail the point here.
Most cables specify total capacitance (Intrinsically Safe or otherwise) rather than wirewire/wire shield. In my experience, the wirewire capacitance should be relatively small to be irrelevent. If it isn’t - maybe due to extensive cable length other factors would have to be also considered; such as cable inductance and frequencies encountered in the cable operating simultaneously.
I'm not questioning the reason you are providing calculations using wire parameters!
In my opinion meeting safety requirements the onus is on the mfg providing the 'component' installed in the IS area!
Phil Corso
Mindful we had been producing IS Designs for many years - one apparently awkward end client [before commissioning] asked us to 'prove your IS Design is in fact Intrinsically Safe'.

Once you have done the calculations you realise in hindsight:
- never assume IS rated devices are safe
- the hard part is getting sensible data from Manufacturers

The satisfaction is the happy client and a proven safe design.
There's a nice app from Endress+Hauser named ExiCalculator that makes calculations based on their sensors models plus cable features. It also print a pdf with results. Just with their products. Downloadable for free from App Store or Google Play
Hello Marat,
I think GE advice is correct: Ccable=Cwire-wire + Cwire-shield
You are taking into account the total energy of the wire contained in its electric field.

In IEC 600079-25:

9.3 Electrical parameters of cables
The electrical parameters (Cc and Lc or Cc and Lc/Rc) for all cables used within an intrinsically
safe system shall be determined according to a), b) or c):
a) the most onerous electrical parameters provided by the cable manufacturer;
b) electrical parameters determined by measurement of a sample, with the method of testing
electrical parameters of cables given in Annex G;
c) where the interconnection comprises two or three cores of a conventionally constructed
cable (with or without screen): 200 pF/m and either 1 μH/m or an inductance to resistance
ratio (Lc/Rc) calculated by dividing 1 μH by the manufacturers specified loop resistance per
meter. Alternatively, for currents up to Io = 3 A an L/R ratio of 30 μH/Ω may be used.
from post #8
"Once you have done the calculations you realise in hindsight:
- never assume IS rated devices are safe"

Are you implying that the agency certifying the I/S device is not doing its due diligence? Or something else? What makes an I?S device installed per its control drawing unsafe?
It's nothing to do with incompetence or due diligence - it is about forwarding IS designs to your client where each device originates from a different manufacturer. While each may have there own disposition, it is for the design company to be absolutely certain what they are selling is IS. Supporting calculations are the easiest way forward.

IS barrier manufacturers provide appropriate data - but how many instrument specifications include fail-to-danger statistics ?

Very few do and it's up to you [as designer) to second-guess using calculations.

Mechanical engineers always used to make comparisons to motor car, so here goes.
In most modern cars, if the engine fails you lose footbrake (and steering) so fails to danger. On a straight level road you coast to a stop - and you have the handbrake. So Safe.
If you are going downhill at some speed and there is a corner right in front of you - you've lost it !
You've now reminded me of self-certification back in the 1990's of an explosion-proof interface box.

It looked right with the correct materials, shape, 'knockouts', no manufacturing defects etc. but you had to check every possible scenario where it would be used.
You had to be sure before signing the Certificate.

After I signed it off - won't be safe in the epicentre of a Nuclear Explosion will it ?
You have to apply a careful balance between misuse and common sense.
Capacitance is strictly important when dealing with high voltage. With 24VDC current instrumentation wiring the risk of arcing is drastically smaller. If you are seeking explosion proof installations, even the maintenance procedures and proximity to dangerous substances are involve. Even then you must be safe, with a safety watch close at hand.