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Topic Title: Zener Barrier Earthing
Topic Summary: Hazardous Area Earthing
Created On: 14 December 2010 04:03 PM
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 14 December 2010 04:03 PM
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GMAC

Posts: 64
Joined: 12 September 2006

If I am fitting zener barriers in an electrical panel in the safe area, I believe the barrier earth has to be connected to the panel main earth terminal via two separate cables.
If the Intrinsically Safe circuit in question is an input to a PLC, fed from a 110Vac input /24Vdc output Power Supply, which is fed from a 230V/110Vac transformer, then would the two earth cables from the Zener barrier earth need to be connected to the neutral leg of the transformer as the transformer secondary is the source of the voltage eventually going out to the hazardous area, rather than back to the panel main earth terminal?
 14 December 2010 04:16 PM
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StewartTaylor

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The straight answer is emphatically NO.

Not only that, the IS barrier should not be earthed to the standard panel safety earth but via a clean connection to a low-impedance true earth connection. The voltage build-up that is permitted on a standard safety earth under fault conditions is greater than the potential with respect to earth permitted for IS applications (about 3V without very low current limit).

The object of a zener barrier is to blow its internal fuses if the voltage relative to earth exceeds the barrier rated voltage. Up to this voltage the internal resistors (typically about 400 ohm for a 24V barrier) keep the potential field current within safe limits.

MTL website has a lot of good information about application and installation of zener barriers.

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 15 December 2010 12:05 PM
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GMAC

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Thanks for reply.
I presume from your answer we are looking at a fault occuring elsewhere in the distribution system which could raise the voltage level of the zener barrier earth if it was connected locally to the panel earth.
If the zener barrier is mounted in a compartment in an MCC then how far back in the earthing system do we need to go to physically connect the zener barrier earth.
Would it be, for example the main earth point in the incomer compartment of the MCC?
 15 December 2010 09:19 PM
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StewartTaylor

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I can't really give you much guidance on that one. In my line of business (petrochem) we run a separate earth from the actual earth rods, mat or whatever it is on the specific site. Sometimes the IS system has its own completely independent earth with its own earth rods.

I know this isn't much help if you're only wanting to install one or two loops but the basic principle is that the impedance to earth should be low enough that with the maximum fault current that can arise on the safety earth the potential at the point where the IS connection ties in should be kept below the acceptable limit (I'm a bit vague on the actual value, but as I said before it's on the order of about 3V).

The other point to consider is that a safety earth for equipment can be a source of noise into measuring equipment.

Another question, if you've got a problem finding a clean earth, is do you need IS? For example, if it's a motor temperature measurement (I'm guessing that, since it's into an MCC) then it probably wouldn't need to be IS if, for example, it's in the motor coils and powers down along with the motor. In fact, unless the necessary level of isolation is preserved between power and IS throughout the loop then other techniques should be used.

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 15 December 2010 10:09 PM
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chartassuk

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Gmac,

Stewart is correct.

For more reading look up technical papers in the support downloads section of the MTL website, specifically:

TP1121 - A definitive guide to earthing and bonding in hazardous areas

Also, where are you fitting the IS equipment in the panel?
 16 December 2010 03:24 PM
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GMAC

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The particular application is in relation to a sewage works, where the MCC supplies power to, and controls various pumps. In addition to the pumps there are float switches in a zone 1 area connecting back to the MCC. The zener barriers are in a control compartment of the MCC, separate from the pump starters. (Form 4 type MCC). The MCC is fed from a main switchboard on the site, which is fed from the site substation transformer.
Typical zener considered for use is an MTL 7787+.
Question is, do I keep the barrier earth isolated from the general steelwork of the MCC and connect it back to the MCC main earth in the incomer section, which comes from the main switchboard, eventually finding its way back to the substation transformer secondary star point?
 16 December 2010 03:42 PM
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StewartTaylor

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I would certainly keep the IS earth isolated from the cabinet steelwork.

I'm concerned about he use of transformer secondary star point as your IS earth point, though. In this case I'd say you might be better to drive an independent earth rod for your IS.

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 18 December 2010 11:37 AM
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chartassuk

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Some points for you to consider Gmac, apologies if you're already aware.

Intrinsic Safety systems should comply with BS EN 60079-25 Explosive atmospheres. Intrinsically safe electrical systems. Latest edition 2010.

This details design, selection, installation and verification requirements.

Some salient points:

Descriptive System Document (DSD) to calculate if the system meets the conditions for IS

Block Diagram showing all components

Separation/segregation of IS circuits (BS6739:2009 also applies for cabling to instrumentation)

Competent installation verified by completion of BS EN 60079-17 installation certificate (also available from NICEIC) by Qualified Supervisor

One last consideration: Are the float switches installed for normal process control or are they required to perform a safety function?
 19 December 2010 09:04 AM
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GMAC

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Many thanks for your guidance in this very important subject (Stewart Taylor and Chartassuk).
I think this thread will help people in the field and the panel manufacturing business obtain a clearer understanding of what needs to be achieved.
Currently I am reviewing the documents you have suggested (in particular the MTL TP1121 document) so that I can get to a point of knowing what physically has to be wired/connected both in the field and the panels.
One further question - On the subject of the float switch in the hazardous area (this is being used as a back-up for a level control instrument if the instrument fails - failure result would be sewage overflow into undesirable areas), should the screen/armour of the float switch cable be connected to the zener barrier earth? If so, what does this achieve?
 19 December 2010 02:10 PM
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chartassuk

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Hello again Gmac,

For cabling, this should be BS5308 'blue jacket', which comes in 4 flavours: part1/type1; part1/type2; part2/type2; part2/type2

part 1 are PE cables mainly for petroleum applications
part 2 are PVC cables for industrial applications
type 1 are unarmoured
type 2 are armoured
also the much less used type 3 lead insulated cable
plus individual or collective screening arrangements

From your description it would appear that armoured cables are being installed, therefore a brass gland such as a CW would earth the armour to the panel termination chamber metalwork as long as paint scraped off / star nut / banjo applied. For IS applications it is not necessary for the gland to be Ex, but should be suitable for environment.

Following BS6739 it is advised that these cables be installed with a segregation distance of 300mm from other types, so possibly on their own containment.

For the screen and drain wire, MTL make the 7799 dummy barrier which earths these cores to the IS din-rail.

Hopefully all your IS equipment in the control cubicle is separated by at least 50mm from other circuits and cabling run in clearly identified light blue cable duct.

On the subject of the float switch, this appears to have an environmental safety function requirement, has anyone done an EIL (environmental equivalent of SIL - Safety Integrity Level) assessment?
 20 December 2010 08:49 AM
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GMAC

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Doesn't the armour need to be isolated from the panel general earth and connected to the separate IS earth rail?
Our earlier discussions on keeping the general earth system out of the hazardous zone so that under possible fault conditions an uncontrolled voltage level may enter the hazardous zone via the general earthing.
Couldn't the armour do so if connected to the general panel earth?
 20 December 2010 08:59 AM
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StewartTaylor

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Generally you would connect the armour to the plant safety earth and isolate it from the IS earth. The IS circuit insulation then has to take care of any differential voltages, but it's not really practical to ensure that cable armour stays isolated from plant earth.

Just a thought, but if you're having a lot of problems with earthing a galvanic barrier might be simpler. You need a power supply for it, but the IS earth is no longer required.

Also, at the risk of insulting your intelligence, you need to make sure that your float switches have suitable contacts (usually precious metal) for the very low voltage/current wetting available with IS.

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 20 December 2010 10:15 AM
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GMAC

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Stewart,
You identified earlier that the plant earth could be at a voltage unacceptable to the hazardous area.
Since the armour of the float switch cable will be running into the hazardous area and as you suggest, connected to the plant earth at the gland, doesn't this mean we could have an unsafe voltage on the armour in the hazardous area?
 20 December 2010 10:22 AM
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jimmy277

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Originally posted by: StewartTaylor

I can't really give you much guidance on that one. In my line of business (petrochem) we run a separate earth from the actual earth rods, mat or whatever it is on the specific site. Sometimes the IS system has its own completely independent earth with its own earth rods.



I know this isn't much help if you're only wanting to install one or two loops but the basic principle is that the impedance to earth should be low enough that with the maximum fault current that can arise on the safety earth the potential at the point where, Duplicate Files are the IS connection ties in should be kept below the acceptable limit (I'm a bit vague on the actual value, but as I said before it's on the order of about 3V).



The other point to consider is that a safety earth for equipment can be a source of noise into measuring equipment.



Another question, if you've got a problem finding a clean earth, is do you need IS? For example, if it's a motor temperature measurement (I'm guessing that, since it's into an MCC) then it probably wouldn't need to be IS if, for example, it's in the motor coils and powers down along with the motor. In fact, unless the necessary level of isolation is preserved between power and IS throughout the loop then other techniques should be used.


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Edited: 20 December 2010 at 10:29 AM by jimmy277
 20 December 2010 11:25 AM
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StewartTaylor

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Accept your point but all the steelwork etc. around it should also be bonded to the plant earth, so the critical thing here is to avoid the possibility of a spark. Basically, the armour is part of the electrical installation of the plant.

Othe point here is that armour should be earthed at one end only, and this can be the field end - not usual, I'll admit.

Are you using cables with screens as well as armour?

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Reality is that which, when you stop believing in it, doesn't go away.
 20 December 2010 03:21 PM
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chartassuk

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Alternative approaches:

a) If the cabling type were changed to unarmoured and then adequately mechanically protected then concern over grounding of armouring disappears.

b) Or, referring back to TP1121:
"Where armoured cable is used for intrinsically
safe systems, then it is acceptable practice to
regard the armour as primarily for mechanical
protection. It therefore becomes part of the plant
structure and hence can be multiple-bonded.
Bonding is usually achieved by using
conventional glands which connect the armour
to the structure whenever they are used. This
does mean that the armour will carry a part of
the current which flows in the structure."


There's an old but good discussion on earthing over on eng-tips, link > http://www.eng-tips.com/viewth....cfm?qid=142945&page=7
 06 March 2011 01:00 PM
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mrshaunsmith

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I can't see that someone has not suggested using Galvanic Isolators instead as this removes the earthing problems. This application is for pump backup so use a galvanic barrier with relay volt free contacts also remember that if the earthing system is TNCS ie pme then zener barriers are not allowed.

Shaun
 13 March 2011 08:04 PM
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fredman

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Joined: 26 July 2007

Couple of points.
Cables do not need to have Blue sheath all though preference, they must be marked that they are IS.
The IS earth on a ZENOr barrier is a safety earth and in general should be taken back to the main earth termination.
Use galvanic barriers, these alleviate all these issues. If you are on a sewage site then sometimes getting a good earth can be an issue, a bad earth is dangerous.
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