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Junction box segregation between DI and DO signals
Junction box segregation between DI and DO

Dear all,

My scenario is I have few nos DI (eg, open close valve) and also DO (110vdc solenoid valve). Im just wondering if there's any standard/reference for junction box segregation between digital input (DI) and digital output (DO) signal or we can share them together in one jb?

Appreciate your advice

2 out of 2 members thought this post was helpful...

There is no such standard to define this. but by experience it should be separated because it having huge potential diff. in DI (8 VDC) And DO (110 VDC).

> There is no such standard to define this. but by experience it should be
> separated because it having huge potential diff. in DI (8 VDC) And DO
> (110 VDC).

Hi.thanks for the reply. But what if DO (solenoid valve) is part of ESD system?Should it have its own JB?

Many thanks

By Bob Peterson on 30 May, 2013 - 10:05 am

In general, there is no problem with sharing the j-box.

There could be some issues with local or plant standards and/or practices.

the devil is always in the details.

1 out of 1 members thought this post was helpful...

As Bob Peterson says, in general, there is no problem with sharing the junction box for DIs and DOs. Many times they are both at the same voltage level.

The real problem with signal level separation/segregation is with cables running in parallel over long distances, especially when one or more of the cables contains conductors which carry medium- to high levels of current, and/or switch the current on and off repeatedly. An example would be running DI wiring (say for a pressure switch) and motor space heater wiring and motor power wiring in the same conduit or cable tray over a long distance. Even if the motor power is single-phase 120- or 208 VAC, if the current exceeds 5 Amperes and/or is switched on and off repeatedly, there may be voltages induced on the DI wiring which can cause problems for the control system (the dreaded "electrical noise"). Or, if instead of a pressure switch a pressure transmitter is used and the 24 VDC/4-20 mA wiring is run in the same conduit with the motor space heater wiring and the motor power wiring. It would be worse of the motor power wiring was three-phase AC, 440-480 VAC.

In control panels and junction boxes, if cables with different signals are run for short distances (a couple of meters or less) in parallel with each other it usually doesn't cause a problem. The problem is when the cables entering or leaving the junction box or control panel are run in parallel with each other in conduit or cable trays over long distances.

The original poster neglected to state the voltage/current levels of the DI wiring, which might make a difference. Also, the current drawn by the 110 VAC solenoids might be a problem, particularly if it's cycled on and off at a high rate. In this case, it might be prudent to use some kind of metal divider between the high and low level sides of the junction box, and, again, to avoid running the conductors in parallel with each other as they enter or leave the junction box.

So, the real problem is not so much with mixing signal levels in the same junction box or control panel, it's with avoiding running the cables entering or leaving the junction box in parallel with each other in the same conduit or cable tray over long distances, particularly if the currents are high and/or are switched on and off quickly.

Hope this helps!

As for a standard, there are many local technical regulations and standards in place in many parts of the world which should be consulted. Also, some plants or companies have their own standards which must be observed. Unfortunately, all too often those requirements don't make it into the bid specifications, and the supplier/installer doesn't find out until after the installation is under way or complete. So, ask, ask, ask, and consult, consult, consult. And still there will be the occasion when you have done your best and you will still find out after the fact about some requirement which wasn't clear or wasn't included in the bid specification.

If you are asking about electrical noise causing problems on a piece of equipment being installed/commissioned, consult the bid specification for requirements. If there are none, well, lesson learned. But, again, unless the junction box is very large and the currents of the DOs and DIs is very high and switching frequently and the cables are run in parallel haphazardly in the junction box, that still might not be the real problem. Cables run in parallel into and out of the junction box for long distances, or even with other cables carrying high voltages and currents, could be the cause of the problem.

Hi sir,

Thanks for the brief explanation. As for local standard, 110VDC solenoid valve is considered under signal class level 5 which shall be segregate from other instrument signal (class 4 and below). If this the case, do you think it should have its own jb? I'd appreciate your expert advice over this matter.

> Thanks for the brief explanation. As for local standard, 110VDC solenoid
> valve is considered under signal class level 5 which shall be segregate from
> other instrument signal (class 4 and below). If this the case, do you think
> it should have its own jb? I'd appreciate your expert advice over this matter.

Sir, where i will get this signal level classification?

> Sir, where i will get this signal level classification?

I'm referring to Shell DEP for instrument signal lines. Hope this will help.

regards

Well writen and explained. Thanks CSA!!!

Induced voltages can cause havoc in a plant and have the technicians running around for weeks on end trying to pinpoint the problems since most of the time these induced voltages could affect more than one signal and will have a intermittent effect on more than one device making it a nightmare to find.

Doing things right from the design phase, working on the highest of design standards when doing modifications in order to keep low voltage instrumentation DI, DO, AI, AO signals separate and away from high voltage cables, is and always will be the best solution to induction problems.

>In general, there is no problem with sharing the j-box.

> There could be some issues with local or plant standards and/or practices.

> the devil is always in the details.

Hi Vijay.

Thanks for the reply. I understand that no issue sharing for AI/AO within the same jb. But for DI and DO, I've experience that some plant have no issue sharing jb for this signal so I thought maybe there is standards that I can refer with.

Regards

It is better to separate the JB if you have different voltage level.

I found the following resource regarding the Junction box:
http://instrumentationportal.com/2011/instrument-glossary/instrument-glossary-j/junction-box/

Hi Andy,

Thank you for the link.

Regards

read this:

http://instrumentationbox.com/junction-box-j-b/