Hi all,

recently I have seen more projects that state the cable screen should only be connected to ground at one end. Up until now the screen was grounded at both ends and we've never had any problems. Just wondering what experience people have had.

Cheers.

One end only. Grounding at both ends will create a ground loop ( a circuit for current to flow, this you do not want)

Have you had any experience of current flowing in a grounded circuit? In my previous job we had 100s maybe 1000s of devices with screen grounded at both ends with no problems on any circuit. What about VFD cables are these to be grounded at one ond only too? In what instance would a screen become a current carrying conductor unless under fault conditions? Wouldn't a screen carring current trip the RCD? Sorry if i sound naive.

Thanks for reply.

Yes I have seen problems with grounding at both ends. Noise, I have not measured the current but once one end was removed the problems went away. ( I have seen the noise on a scope)

I have no experience with VFD's so cannot answer that question.

Point "A" ground and point"B" ground can have a difference of potential. The greater the distance between the two the greater the possibility . Ground is a very noisy environment. Current will flow.

Sorry, I'm not familiar with the term "RCD".( residual current device (RCD)? I have never seen current protection on a shield wiring.

See link below for some reading material on the subject.

http://literature.rockwellautomation.com/i...rm001_-en-p.pdf

Thanks for replies Mickey,

I was under the impression that if there was curent flowing in the earth circuit (above 30mA) that the supply RCD would trip, several of our panels have RCD protection on the supply and we've never had any problems. We might just have been lucky.

Cheers

I agree with the noise point.
Would also say that the grounding should be done at the supply( panel end) , if the cable run is to an external piece of kit. You will still have protection in case of earth fault if the cable gets damaged .
Dont know of the technical bits but this is the practice we adopt.

RCD's measure the current differenece between L + N not the current flowing in the grounding circuit.
The current in the grounding could come from various sources, none of which are individually high enough to give a fault.


I'm for full 360 degree screening at both ends.
This is usually to a big lump of metal that acts as a parallel earth.
Mostly on encoders or comms.

I've had problems with encoders (lost pulses) where the screening was not complete.

One end screen seems mostly to be favoured by 'old school' people.

If I thought there were going to be problems (due to the distance bewteen the screen points for example) I'd look at adding or changing something rather than taking away one of the screened ends.

I'd only connect at one end if the manufacturer recommended it.

I'm not sure what items people have had trouble with screened at both ends.
If removing the screen at one end works try removing screening at both ends and see if there is a difference.

I recommend grounding both ends but always verify with the manufacturer's specs.

I recently had issues on with ModbusRTU where a VFD was causing interferance. I took the time to read the manufacturers recommendations (Omron in this case), connected the drain at both ends instead of one, and the problem was fixed instantly.

On VFD cable always connect both ends of the screen to ground. All the drives I've worked with spec this, and from experience it is absolutely necessary if you want to eliminate / minimize harmonic polution.

Thanks for replies all,

Veganic, I'm not 100% positive how RCDs work, I know the theory of difference between currents in N and L but we have RCDs supplying three phase motors (mennekes plug sockets from RS http://uk.rs-online.com/web/search/searchB...&R=0491204) with obviously no neutral don't these monitor current flowing in the earth circuit?
Again, sorry for my naivety.

Thanks

It depends. There's grounding, and then there's grounding. If you are talking about power circuitry, then BONDING is required. If you ground at multiple locations, you can create a distributed ground. This MIGHT be OK in some instances but it is generally a very good idea to avoid this if at all possible. The infamous "stray voltage" problem that you may hear about is because utilities commonly use distributed grounds (ground everywhere it is feasible to do so) and the results have been true ground loops with voltages and currents high enough to cause problems with livestock and various other strange situations. The following web site explains how/why this occurs towards the end with utilities that violate the basic rules on isolating grounds:
http://www.bassengineering.com/SV_Cause.htm

That's not the best ground anyways. I currently work in a mining company. We run 7200 volt and 4160 volt equipment for the most part. Even the usual safety function of proper grounding and bonding (tripping the overcurrent protection device in the event of a ground fault) simply doesn't work in many cases because the fault current is insufficient to trip anything. At 7200 volts if the line impedance is 10 ohms, the fault current is 720 amps. That's OK except when you have a 1000 A piece of mining equipment...the circuit breaker will never trip. In addition, achieving an adequate Earth ground isn't easy with the soil around here (very sandy and dry). So we run high resistance grounds with ground fault detection relays (to handle the low fault current issue) and try to do the best we can with bonding and grounding to Earth ground, which is much less critical in a high resistance grounding system. All equipment is connected to a 4-wire system (to provide the lowest resistance bonding paths we can get).

If you've never heard of this, check out this article that explains it:
http://www.kilowattclassroom.com/Archive/NeutRes.pdf

As to why you'd want to do it, check this out:
http://www.mikeholt.com/mojonewsarchive/GB...ng~20040812.php

On the signal side of things, if you connect both ends of the shield to a ground, then what you've just done is to create a continuous loop. A loop is a form of antenna, and specifically current loops are optimized to receive magnetic fields...which also so happens to be the same RF noise that is most common in industrial plants. So effectively you've created an antenna and you are coupling it into your signal lines by induction...not a good idea. The gain isn't great but I think some of the numbers I've seen on Ethernet shielded vs. unshielded systems suggest that you can get at least 5-10dB of RF gain sucking in all the available nearby noise. Of course your results may vary because you may or may not have lucked out and experienced this problem and because noise interference is highly situation dependent.

My favorite story on Grounding and Ground Loop currents appeared over 40 years ago in the Hoard's Dairyman as I recall, i was just a wee lad at the time. But as my father read and explained the article to me a farmer in the MidWest was being sued by the Electric Utility for "theft of services". Turns out the right of way across the farmers property was 50 feet wide and 2.5 miles long. This ingenious farmer had erected a row of telephone poles on his side of the right of way and run a 00 wire the length of the right of way. The wire was grounded at one end and grounded at the second end after running thru a motor/generator. The amount of power harvested heated, cooled, lighted and so forth the mans entire house.

For those that wonder the Electric Company lost the suit. The judge ruled that if they could not contain their commodity within the negotiated Right of Way the the salvaging of this lost commodity by the Farmer was not stealing.