FP32 Grounding

[io_learner]

Looking at MTL's document TAN1010 : Lightning & surge protection for
fieldbus systems, it does not explicitly state where the surge protector should be grounded. If mounted inside the cabinet, should the FP32 be grounded to AC or DC ground bus? What are the pros and cons between these 2 busses?

[aagostin]

Basically, the surge protector conducts the high surge current to local ground. The side effect is that this local ground potential rises, due to the current flowing into it. As such, it must be ensured that other equipment connected to the fieldbus wires (e.g. host) are not connected to a completely different ground (unless you use another surge protector on that equipment, too).
Let us make an example; let's say the surge protector conducts a high surge current to ground, and because of this current the ground potential where the surge protector is connected to rises to 4 kV. These 4 kV represent a common mode voltage potential for the + and - of the fieldbus, as they are referenced to this connection when the surge protector conducts.
However, + and - are also connected at the host, and let us now assume the host ground is connected to a totally different ground. The ground potential of the host would then >>not<< rise, or at least not with the same magnitude. Let us just assume that is would rise to 1 kV. As such, there would be a potential difference of the two grounds of 3 kV. The ground potential difference as such does not really bother us, but since + and - at the surge protector is at the high potential while + and - at the host card are referenced to the low potential, we actually get a potential difference at the host's + and - versus the local ground, across the electronics of the host card. As industrial electronic withstands 2 kV, a 3 kV voltage could well damage the galvanic isolation and ith it the electronics of the host card.

There is an addendum to the AN1003 and 1010 which I attached.

Basically, the right grounding location for the surge protector is where also the DCS system is grounded. The shield ground potential itself is not so much of importance compared to the electronics of the host card. Eventually, instrument earth is also connected to the other earths via a star point grounding method.
Most important is that all ground wires are as short as possible as well as thick as possible, to avoid a high voltage potential across the grond wire.

Please feel free to contact me if you require further information.

[io_learner]

Hi Andreas,

Thanks for your comments. In short, if a DCS system employs single point (star point) grounding, then the AC and DC ground busses are at equal potential and it would not matter if the surge protectors are grounded on the AC or DC bars. Please advise if I understood correctly.

Ï would like to clarify on one of your points: "The shield ground potential itself is not so much of importance compared to the electronics of the host card". What is the rationale behind this? Can you explain this further? Usually, the shield wires are tied to AC ground in the cabinet.

[aagostin]

Hi Andreas,

Thanks for your comments. In short, if a DCS system employs single point (star point) grounding, then the AC and DC ground busses are at equal potential and it would not matter if the surge protectors are grounded on the AC or DC bars. Please advise if I understood correctly.
Yes and No; If surge protector and DCS are grounded to different grounds, it will matter if the overall ground wire from the ground terminal of your surge protector to the star point where all grounds are combined is long and has a ohmic impedance of more than 0.1 Ohm. At 20 kA maximum surge current, you would then get 2 kV. Anything higher may cause damage.

If cables are short and low ohmic, it does not matter.
Also, if DCS and surge protector are grounded to the same ground, it does not matter whether this ground is AC or DC ground.
However, always follow the DCS vendors' recommendations. They have extensive experience on their own system and what groundin leads to best results.


Ï would like to clarify on one of your points: "The shield ground potential itself is not so much of importance compared to the electronics of the host card". What is the rationale behind this? Can you explain this further? Usually, the shield wires are tied to AC ground in the cabinet.
What I wanted to say is: Let us say your surge protector on the incoming trunk is grounded to "DC ground", shield to "AC ground". In case of a surge, surge current is diverted to DC ground, letting the DC ground rise. With this, not just the + and - conductors are referenced to DC ground, but also the shield, as they are all held in place (=in reference) by the surge protector components.
Between shield and DC ground, you would have maximum something like 30V (GDT/TranZorb diodes). So if the DC ground rises to 4kV, the shield would be approximately at this potential, too. The AC ground however may only rise to 1kV as in my original example. So across the shield length, you would have 3kV between the shield terminal of the surge protector and the location where the shield is actually connected to the AC ground. This will cause a cross current, meaning potential equalization will take place via the shield itself. This cross current is however significantly smaller than the initial surge current, hence the statement "...is not so much of importance...". This ground potential difference / cross current is unlikely to cause any damage at all, as the wire pairs are twisted and the induced current caused by the cross current is ideally Zero if the twists are ideal (means 100% the same). In reality, twists are not 100% the same, so there is a small induced current, but this is a negligible effect in this case.

[CHRISC1024]

I've always been told that the fieldbus shield and grounds should not be tied to the same bus bar in the control panel as the AC grounds. I have been shown installations with this senario and we have seen significant noise introduced into the FFB signal. The AC and DC grounds are tied to the same ground system but at the earth ground connection, not inside the panel (IE 1 ground cable coming from earth ground jumpered to both the AC and DC ground buses).

It has always been our understanding that the FFB and 24VDC ground be isolated in the control panel and only tied together at the earth ground.

[Dan Dumdie]

Yes, Chris is absolutely correct! This keeps FFB noise to a minimum.