Instrument Grounding Scheme

[tt90lrb]

I recently ran into a client grounding scheme that I'm not too familiar with. I could use some insight from the group.

For instrumentation (pressure transmitters, for example) and communication (serial coms), they connect the sheilds to a cabinet ground buss labled "IE" which I'm assuming is Instrument earth. Equipment grounds are taken to another cabinet ground buss labled "PE" which is protective earth.
This is a very common practice which I'm use to seeing.

However, instead of separate ground cables connecting both busses to external customer PE and IE grounding points, only a PE ground is run. A .01uF is connected between PE and IE in the control cabinet. This is the practice I'm not use to seeing. What are the advantages/disadvantages to this configuration.

Also, I'm told that the individual serial com shield were passed through separate .01uF caps to PE before a single cap was used for all connections from IE to PE. Wondering if this practice was only intended for serial communication and possibly extended (by error) to all instrument grounds.

Your comments are greatly appricated.

 

[dpc]

I haven't seen exactly this configuration, but it appears they are trying to use the capacitors to isolate the IE from 60 Hz voltage while providing a path for high-frequency noise.

What is happening with the shields at the instrument end? Are they grounded at the field end?

 

[tt90lrb]

No, sheilds are not connected at the field end.

It doesn't seem like a solid path to ground on the control panel end (all passing through a ceramic cap). Would this arrangement provide any noise suppression (I guess I'm not sold on the arrangement)?

Main concern is the serial communications lines. Several passing through a crowded noisy location. The client already has communication issues on other pieces of equipment in the same area (provided by other vendors).

 

[busbar]

Might sift through IEEE Std 1100 {emerald book} for supporting text of whatever method you use. For instance, in RS232 loops, the usual 25-pin connector uses a separate signal ground and frame ground, where the usual 9-pin connector only uses a signal ground.

IEEE Std 1143 or 1050 may have applicable information.

How are you reimbursed for debugging? That may influence your favor of customer preferences/”standards” over yours.

 

[dpc]

Are these typical 4-20mA signals?

 

[advidana]

That method was used in old radio record players and radios back in the 30's and 40's and 1950's. The old ac line plug did not have a ground wire just a neutral and hot. The capactor was used in case the plug was reversed. The logic being that the resistance thru the person was higher than the resistance thru the capacitor so you did not feel the shock (as bad)if you touched the metal on the record player or the metal of thru radio chassis if the metal chassis has connected to the hot side. The capactors would sometimes open up then you would get a large shock if you touched the metal chassis. I am sorry to see that method still being used.

 

[tt90lrb]

Talked to one of the more senior engineers. The intent is to block DC and low-frequency currents by connecting blocking devices (such as a cap or zener diode) between the cable sheilding and the PE ground. This would allow current flow in the sheild wire as well to prevent ground loops.

I'm assuming that by "blocking" DC and LF currents, these two events typically do not interfere with most signal communication.

It's still not something I'm use to seeing. The implemenation (a single ceramic cap between the PE and IE busses) seems undersized for the currents that could be generated.

And to answer DPC, most are 4-20mA signals. But the most critical signals are the RS485 communication to field devices. signal converters with RS485 outputs are used to communicate long distance through the customer facility.

 

[dpc]

I'm not quite sure I buy your senior engineer's explanation. A 4-20 mA signal is basically dc. You won't be driving any dc current through that capacitor. And a 0.01 uf cap presents a pretty high impedance to 60 Hz.

I think they were trying to provide a path for high-frequency noise from shield to ground while maintain dc isolation. This may be a by-product of the way the signals are handled in the control system. I guess if it's working for them, I'd cross myself then close the cabinet door and go on to the next problem.

 

[corrosivo]

The IE bus is not a real ground, its the point where all the shields are bonded together to be at the same electrical reference.
IE bus is isolated from the power ground to prevent noise coming from the power system entering to the sensitive control and communication equipment by the common or differential modes.
Remember the shield in the signal wires is generally made of a thin film of aluminum with a thin drain wire, it does not shield against mag. fields only works for electrical fields up to certain degree, the wires inside the cable must be twisted for cancelling the magnetic induced noise. The 4-20 ma loops generally are inmune to this noise, that is the very reason it is used in industrial enviroments. All the data or signal comm. lines must run in steel rigid or flexible conduit in the high electrical power enviroments such as cable trays etc. there is no other solution unless you use fiber optics for the comm. lines but this might be expensive. All the shields must be connected only in the cabinet (common) side to prevent equipment that might be at different "ground" potentials to be bonded together thru the shield causing a ground loop, this is important. The high freq. signals induced in the thin foil shield see a high impedance along the thin foil shield if it is long, also consider the electrical Z of the power ground cable at the noise frequency, so the capacitor is worthless for drainig them too.
If you are serious about grounding, then this IE must be grounded with a separate ground right at the cabinet floor with a grounding bar or grid the grounded wire must be kept short, it must not be bonded together with the power ground, building structure etc., this configuration will keep the shield and power system grounds independent, remember the ground is for personnel protection not for other purposes.
With the present configuration this installation is unsafe, any energized wire can touch the shield in anyplace and will put a dangerous voltage in all the shields in the system and worse; you are not getting the noise inmunity you want.

 

[BrianR]

The central idea in this form of ground connection is to eliminate any common path between power currents and signal currents to ground. The resistance of a common path causes common mode voltages to develop and cross couple into the other system. This is implimented by first providing seperate ground connection points for power, digital signals and analog signals then wiring them to the common ground point through seperate cables within the cabinet.

The shields on the control cables running from cabinet to cabinet should be grounded at both ends to provide effective high frequency grounding however because of load differneces in the power and phase imbalances the ground potential between the cabinets is different. Unless the path for power currents is blocked, the control cable shields will now carry power current which will induce power frequency interference into the control signals.

The function of the 0.1uF capacitors is to break the low frequency path while giving a low-Z ground to the high frequency digital signals.

A very good reference for further reading is: Noise Reduction Techniques in Electronic Systems by Henry W. Ott.