# terminating shield drains in jb



## stuiec (Sep 25, 2010)

Hey guys, I'm building/terminating a number of JBs for I/O between the field and plc cabinet, and have a couple of questions.

All of the cables to and from the field, as well as the plc cab are to have their drains tied together at an isolated gnd bar. Is it common practice to strip the cable jackets right back to the point of entry and then run the twisted pairs through the wireway to the terminal blocks? (2/16,2/18,3/18 cable) The drawings show my gnd blocks up close to the cable connectors.

Also, is it common to heatshrink the drains too? (polycarbonate jbs with steel back pans)


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## RePhase277 (Feb 5, 2008)

I personally would strip them long so I could have a long drain lead, then I'd cut the comm wires back to the jacket/shield leaving them just long enough to terminate. In other words, leave them shielded right up to their termination.

Then run your drain wires to the bar. Heat shrinking would be optional, depending on the mood I was in at the time.


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## bill39 (Sep 4, 2009)

InPhase277 said:


> I personally would strip them long so I could have a long drain lead, then I'd cut the comm wires back to the jacket/shield leaving them just long enough to terminate. In other words, leave them shielded right up to their termination.
> 
> Then run your drain wires to the bar. Heat shrinking would be optional, depending on the mood I was in at the time.


Typically shields for instrumentation are only grounded on one end, so in a JB where you're just splicing (extending) the cable then land the shields on individual ungrounded TBs. Regarding how far to strip back the jacket, I would remove it far enough back to make it look neat. If it is truly just a JB I wouldn't worry about noise being a problem, especially if it is a 4-20mA circuit.


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## stuiec (Sep 25, 2010)

InPhase277 said:


> I personally would strip them long so I could have a long drain lead, then I'd cut the comm wires back to the jacket/shield leaving them just long enough to terminate. In other words, leave them shielded right up to their termination.
> 
> Then run your drain wires to the bar. Heat shrinking would be optional, depending on the mood I was in at the time.



Do you mean that you'd run enough cable in order to strip for an extra long drain, and then run it (drain) back to the gnd block?





bill39 said:


> Typically shields for instrumentation are only grounded on one end, so in a JB where you're just splicing (extending) the cable then land the shields on individual ungrounded TBs. Regarding how far to strip back the jacket, I would remove it far enough back to make it look neat. If it is truly just a JB I wouldn't worry about noise being a problem, especially if it is a 4-20mA circuit.


It is definitely just a jb, all through terminal blocks. The engineer has drawn it up using two larger terminal blocks on which I am to land many sheilds........are you saying that they ought to be on individual tbs?


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## Peewee0413 (Oct 18, 2012)

bill39 said:


> Typically shields for instrumentation are only grounded on one end, so in a JB where you're just splicing (extending) the cable then land the shields on individual ungrounded TBs. Regarding how far to strip back the jacket, I would remove it far enough back to make it look neat. If it is truly just a JB I wouldn't worry about noise being a problem, especially if it is a 4-20mA circuit.


 99 percent of the time there is no problems even if you do it wrong. lol. ..just avoid multiple grounds to prevent loop. I keep all shields separated until grounding. At one end of course. although you might not have a problem having a common shield tb, I'd personally avoid it just in case a 1 percent problem occurs. A device in one area could screw with others. I dislike using heat shrink on a drain, especially in a analog jb. Putting it on a drain doesn't make the comm better, does it? Lol


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## bill39 (Sep 4, 2009)

stuiec said:


> Do you mean that you'd run enough cable in order to strip for an extra long drain, and then run it (drain) back to the gnd block?
> 
> 
> 
> ...


Just don't exceed the maximum number of wires per terminal and don't use the "grounding" style terminals where they are internally connected to the din-rail.


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## KennyW (Aug 31, 2013)

Don't tie all the drains to a single terminal, treat them like individual signal wires and spice them 1 to 1. If you toe them together you create the potential for ground loops across those segments.


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## bill39 (Sep 4, 2009)

KennyW said:


> Don't tie all the drains to a single terminal, treat them like individual signal wires and spice them 1 to 1. If you toe them together you create the potential for ground loops across those segments.


Kenny - I stand corrected and agree with you. One wire per terminal.


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## gnuuser (Jan 13, 2013)

ugh i know what you mean!

i have a hell of a time getting some of the knot-heads to understand the difference between a drain wire and ground wire ( they always bond them at both ends out of habit i guess) 
regardless of if they are journeyman or apprentices I always double check the installs.


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## stuiec (Sep 25, 2010)

KennyW said:


> Don't tie all the drains to a single terminal, treat them like individual signal wires and spice them 1 to 1. If you toe them together you create the potential for ground loops across those segments.


This makes sense to me...though the prints, from the automation company spec a straight up section of ground bar for the drains. This one http://www.ilsco.com/e2wShoppingCat...k=2100001183:3100012157:3100012384:3100012907

I'll have to have a closer look at the cable incoming from the plc cabinet. It is a multi conductor teck, maybe it has a single drain for connection at the plc cabinet?


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## donaldelectrician (Sep 30, 2010)

gnuuser said:


> ugh i know what you mean!
> 
> i have a hell of a time getting some of the knot-heads to understand the difference between a drain wire and ground wire ( they always bond them at both ends out of habit i guess)
> regardless of if they are journeyman or apprentices I always double check the installs.




You are a Smart Man Gnuuser .


A Smart Man .


Pece
Buddha
Don


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## RePhase277 (Feb 5, 2008)

So this is a splice box? For some reason I was thinking this was a terminating end. If this is in the middle of a run, then definitely splice the shield drains 1-to-1.


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## glen1971 (Oct 10, 2012)

Here are some hints how I do mine.. I'm not saying it's right or wrong, but I've never had an issue while doing it this way. Varying from it has resulted in some rework and problems...

Multi pair/triad teck cable in a PLC, MCC or Source end including junction box (i.e. 2Pr to an end device):
- Most belden multi pair/triad teck cables do not have a ground in them. It is a cable shield. 
- All the individual shields are grounded at the PLC, Motor Controller or Source end. Typically I'll take the overall cable shield to the ground bushing, cut it off then run a #14 green to the ground bar. The individual ones are usually bonded to ground at their terminal in the strip, with a bonding conductor taken to the ground bar, or a ground bar, or similar method.
- I leave the foil on as close as practical and tape it off before it's removed in the panduit.
- If you take a multi pair/triad cable out of the junction box to an end deivce, the cable shield needs to be properly bonded in the Junction Box (same as above).

Multi pair/triad teck cable in a field junction box from a PLC or motor RTD box:
- I leave the outer rubber jacket on until half way up the panduit or side of JB. Then I strip the rubber and cut the cable shield off and then put a couple of wraps of tape around the cut rubber just for aesthetics.
- I take a #14 green trough all the ground bushings to the JB ground on the backplate. Usually one #14 per row of connectors.
- Each pair or triad is then prepped using my "measurement" for determining how much foil is left on. See how I do single pair/triads for what this is.
- The shields are all terminated separately to their own terminal. Not a grounded one or two shields under one terminal. Both of these will create a loop.

Single pair/triad teck cable in a field junction box, PLC, or MCC:
- The outer rubber is left on until my "measurement" just before termination. For "my measurement", I use my right index finger for this measurement. Why you ask? Because unless I have a serious accident, that length is readily available and consistent. I then strip off the rubber, and foil and terminate the conductors.

End device:
- No shield in the field.. I'll say it again - No shield in the field. 
- If you are using a standard teck conenctor, when you remove the rubber, cut the shield off and tape the cut on the rubber jacket. 
- If you are using a multi pair or triad cable, remove the foil, cut each shield separately and tape each pair individually. Cut the cable shield once every pair is done and tape the overall bundle. This will prevent any shields from contacting each other and/or ground causing a loop.
- If you are sealing, follow the above steps before your seal and pour with SC4. It is easier to do this cable prep before the connector is installed on the cable.

Splicing shields in a fitting:
- If you need to change to a teck cable from a conduit system with belden and need to splice the shield, it can't touch the ground here either.. 
- I keep this splice as short as possible and use 1/16" or 1/8" clear shrink on it to keep it isolated and 29 marrettes..

If you are using belden wire, follow the same guidelines as the "end device" and "single pair/triad" terminations.

Some specs will want the shield into the end device. I have no idea why, but I've used use clear (or spec'd color) shrink and bring it into the device, with the end of the shrink squeezed flat. They may also require the foil to have clear shrink installed over it up to the point of termination to keep it intact. If this is the case, keeping the rubber on as far as possible is a huge labour and shrink saver... Read your specs before you start!!

If you are in doubt if your shields are grounded and it is causing an issue, start at the PLC, MCC or source, disconnect the shield and meter it to ground. If you have taken care throughout the whole process, they should read "open" on continuity.. If they don't move to the JB and check from there out...

Guys that take short cuts on shields or say "it doesn't matter about grounding them" have never had to troubleshoot phantom alarms and erroneous analog values because of it.


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## John Valdes (May 17, 2007)

I don't see why you are terminating in the junction box to begin with?
If the cables are to travel further to their connection point, why not just pull them through?
What am I missing here?


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## stuiec (Sep 25, 2010)

John Valdes said:


> I don't see why you are terminating in the junction box to begin with? If the cables are to travel further to their connection point, why not just pull them through? What am I missing here?


 There are three multi conductor teck cables from the Plc cabinet to a number of jbs. From the jbs many smaller (fewer pairs) multi conductor cables run out to field devices.


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## stuiec (Sep 25, 2010)

Is there any reason I should not add a two-tiered through TB every second block to land the drains? Seems it would make for a simple situation stripping wise...


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## bill39 (Sep 4, 2009)

stuiec said:


> Is there any reason I should not add a two-tiered through TB every second block to land the drains? Seems it would make for a simple situation stripping wise...


Or use a 3 level terminal like an AB 1492-WTF3andsave even more space.
http://www.ab.com/en/epub/catalogs/12768/229240/229268/3170951/2297059/Sensor-Blocks.html


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## glen1971 (Oct 10, 2012)

bill39 said:


> Or use a 3 level terminal like an AB 1492-WTF3andsave even more space.
> http://www.ab.com/en/epub/catalogs/12768/229240/229268/3170951/2297059/Sensor-Blocks.html


I've never been a big fan of them... Most panel designers don't allow enough room in their panduit for a loop of any kind for the poor field electrician to terminate with.. More terminals would lead to more wire and way less room in the wireway.. IMO....


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## stuiec (Sep 25, 2010)

So its been into commissioning at this plant for awhile now, and the drains are just beginning to be an issue. I spoke with, and documented via email my concerns with how they were set up, so its all extras now. Its only some of the 4-20ma ones (big surprise), that are acting up. Glad I covered my butt, and thanks for the help fellas.


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## glen1971 (Oct 10, 2012)

gluiec said:


> So its been into commissioning at this plant for awhile now, and the drains are just beginning to be an issue. I spoke with, and documented via email my concerns with how they were set up, so its all extras now. Its only some of the 4-20ma ones (big surprise), that are acting up. Glad I covered my butt, and thanks for the help fellas.


Good luck!! Glad to hear you have documentation to CYA... Now the fun begins to find and fix it... Maybe someday clients will understand it is cheaper to do it right the first time....


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