Field wiring- Segregation of IS cables and other cables

Recently there has been concern expressed about the possibility of inducing unacceptable levels of energy in intrinsically safe cables from power cables and it seems opportune to review current practice in light of these concerns.

The UK view [and possibly the international view] on this subject was determined many years ago and was largely based on research done by ERA. There is a paper; “Project 3051 Induction in intrinsically safe circuits” dated October 1974, which is still probably the best reference on the subject. The paper does not indicate the authors but from memory was probably a combination of H Riddlestone and A Bartels so has a high level of credibility. Basically the paper argues that the principal problem is magnetic coupling from low frequency [50Hz] high current power supplies. It is argued that higher frequencies use lower currents and require higher values to produce ignition and hence are not the primary risk.

In practice the need to avoid interference problems decides the layout of instrumentation cables in general and IS cables in particular. The majority of instrument systems are adversely affected by interference at levels well below the energy levels necessary to cause ignition.

It is normally considered good practice to run instrument cables in separate cable trays from power cables. From an IS viewpoint this has the advantage of making inspection and maintenance a much better defined task. If IS and non-IS instrument cables are in the same tray they are usually segregated by securing them to opposite sides of the tray or using dividing plates. [ IS cables are frequently identified by using a bright blue outer sheath. The code of practice states ‘light’ blue but the colour used would not be recognised as light blue by anyone from Cambridge University].

The majority of instrument cables have an outer screen and use twisted pairs, which has the dual effect of reducing emissions and preventing interference from entering the system. The code of practice [IEC 60079-14] suggests that if either the IS or non-IS cable is screened or armoured then no segregation is required. In practice the IS cable is normally screened and the power cable is normally armoured and hence there are two mechanical barriers between two circuits and hence interconnection is very improbable. If a power cable is carrying both supply and return balanced currents then the magnetic field generated is not large and this is further reduced if the cable is armoured. There is no requirement to use twisted pairs in the instrument cable but this is beneficial and practicable hence is recommended. The toughened outer sheath frequently used for Exe power circuits is an adequate mechanical barrier but is not included in the acceptable single layer protections, presumably because it has no electromagnetic shielding effect

From an IS viewpoint there is no requirement for segregation from power cables of screened IS cables but for interference reduction reasons avoiding long parallel runs and some segregation is desirable. The advice on segregation distance varies but even small distances [5cm] are beneficial and a general guide of greater than 50cm has a number of advocates.

To summarise, if a belt and braces approach is feasible, use a separate cable tray for instrument cables, and use cables with an outer screen and twisted pairs. The power cables should be armoured and fixed to a separate cable tray at least 50cm away. It is unlikely that this ideal situation can be maintained throughout the cable runs but they can be safely brought together for short distances. Suitably protected power and IS cables can be safely run in close proximity if this is unavoidable but this is not desirable for interference avoidance and clarity of identification reasons.

An Afterthought. The code of practice warns against exposure to intense magnetic fields and cites proximity to overhead lines as an example. The existence of strong magnetic fields in a hazardous area [hopefully only a Zone 2] would presumably cause significant currents to flow in any conducting object for example handrails. Presumably the risk analysis of the plant would ensure that any IS cables were protected from such fields possibly by enclosed metallic trays. Making sure the trays are non-incendive is an interesting problem. It is interesting to speculate what faults in an overhead line have to be taken into account when doing the risk analysis.

Explore posts in the same categories: Explosion Protection, Hazardous Areas, IEC60079, Intrinsic Safety

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