armor fieldbus cable

[VickyWu]

Hi
According to Relcom/MTL fieldbus wiring guide, “if the fieldbus cable is protected by armor, it is normal practice for the armor to be electrically connected to local plant metalwork at each end.”
1. why it should be electrically connected to local metalwork?
2. what means at each end? normally it is done by JB (one end?)
3. If the armored fieldbus cable is not electrically connected local plant metalwork then what possible problems are?
Please advise.

thanks

[marisg]

Vicky,

To understand my answers to your armor grounding question, I need to give you some background information. Unless a plant is very small and the metalwork in it is very well interconnected, there are voltage potential differences between parts of the plant. In other words, there is no such thing as a GROUND. If the fieldbus cable shield is connected to the local “grounds” at both ends, there will be current flow in the shield and introduce ground loop noise. For this reason, the recommendation is to connect the cable shield to the local ground in only one place, usually the control room end. This method, however, is not totally ideal. There are other sources of noise that could be minimized by shield grounding at both ends. Armored cable provides this opportunity. The metal armor is electrically isolated from the cable shield. If the armor is grounded at both ends, it provides an additional shield for noise protection.

[Rockbestos]

Vicky,

Grounding armored MC cable is described in the NEC article 250.118

(10) Type MC cable where listed and identified for grounding in accordance with the following:
a. The combined metallic sheath and grounding conductor of interlocked metal tape–type MC cable
b. The metallic sheath or the combined metallic sheath and grounding conductors of the smooth or corrugated tube-type MC cable

The armor should always be connected through the gland to the Junction box. If possible, grounding both ends provide better grounding.

[Mike ONeill]

The armour (1) may act as a secondary power return conductor for fused grounded neutral supply systems (which is why it is required to be grounded) and (2) eliminates direct touch hazards through elevated ground potentials on powered equipment (which is why it is grounded at all points of connection).

If you don't ground the armour at, say, the field device, then it is possible that under fault conditions, you could develop a dangerous potential between the device case and the cable armour. This could be a touch hazard for a technician or a spark hazard in a hazardous area. Say it was a temp tx on a big motor. The 3-phase windings are not perfectly balanced and some stray current flows through the ground connection on the motor and also through the motor casing. If the cable armour was exposed (and if it was steel wire armour via a cable gland, the gland body is connected to the armour when you install it), then there could be a voltage between the motor case and the tx body.

Remember, there is no such thing as zero volts; all voltages are relative and they are relative to whatever you are measuring between. Something labelled ground could be at 250Vrms if there is no continuity between that ground conductor and the return common point of the ac supply. So to make things as safe as possible, the armour is grounded wherever it is accessible and to local metal work along the way. You can only blow a fuse if you can draw current, and you can only draw current if there is a path. It may be unfortunate if you or someone else happens to be the connecting link in that path.

The shield is a different story - stray power currents are not encouraged to flow in the shield and they aren't designed to carry current anyway. The shield literally drains away very small currents which are usually capacitively coupled from noisy sources, thus preventing them getting onto the signal wires. In order to achieve this, the shield is only grounded at one end (by convention and for convenience, at the DCS end) and left floating (UK/US) or grounded through small caps (Germany) at all other places.