Why Separate Junction Boxes When Transfering 4-20 ma and 230 AC Signals

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Thread Starter

Shailesh09

I have to transfer following signals to DCS.
1. 4-20 mA Pressure signals
2. 24 V dc solenoid signals
3. 230 V AC signals like start/stop commands for motors.

how many Junction boxes should I consider?

please revert back on [email protected]

 
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Bob Peterson

there is no requirement to have any junction boxes at all. they are for convenience.

most times people tend to separate signals by voltage level, either into different boxes or different areas within the same box, but at least in the US the electrical code does not always require it but depending on voltage levels and insulation ratings an what class of conductors the wires are classified as you might need some separation.

IMO, most of the time these kind of conductors are class 1 conductors and thus can all be run together as long as the insulation rating is greater than the highest voltage on any of the conductors.

Whether this is a good idea (for noise or transients or whatever) is a design decision and not a code issue.

--
Bob
http://ilbob.blogspot.com/
 
Minimum of 2 junction boxes.

The 230 VAC home run cable will be different from the 24 VDC. You can mix analog and discrete low Voltage DC signals in the same cable / J box without a problem. but be aware that large coils such as solenoids might interfere with inputs as they switch, may need to add a suppression device.
 
Hi, Bob.

IMO, 24VDC and 4-20mA are more typically Class 2 circuits. 230VAC signals are definitely Class 1.

In the U.S., we install these to the requirements of NEC 725.136 (that's Article 725, Section 136).

In general, cables and conductors of Class 2 and Class 3 circuits shall not be placed in any cable, cable tray, compartment, enclosure, manhole, outlet box, device box, raceway, or similar fitting with conductors of electric light, power, Class 1, non-power-limited fire alarm circuits, and medium-power network-powered broadband communications circuits unless permitted by 725.136(B) through (I).

Please see 725.136 for additional detail.
 
W
A minimum of two junction boxes would be required. As already discussed, you can divide the voltage levels up and separate them as appropriate (See IEEE 518). Also, see the website below:


Cable spacing as a means of noise mitigation: http://electrical-engineering-portal.com/cable-spacing-as-a-means-of-noise-mitigation>
electrical-engineering-portal.com.

In situations where there are a large number of cables varying in voltage and current levels, the IEEE 518-1982 standard has developed a useful ...

You should as a minimum separate the 4-20 mA/24 VDC from the 230 VAC circuits for two reasons. The first reason to keep the 230 VAC separate is that the AC can easily propagate on to the 4-20 mA/24 VDC circuits via capacitive coupling (switching DC solenoids can also propagate noise as already mentioned).

The second reason to keep the 230 VAC separate is a potential health hazard, e.g. that level of voltage can easily kill a technician under the right circumstances. You do not want to mix that level of voltage (or any other AC voltage for that matter) in with 24 VDC level circuits because the instrument tech/electrician may be working a 24 VDC circuit and inadvertently get on the 230 VAC getting the shock of his life, maybe the last one he will ever get. Putting them in a separate, clearly marked junction boxes will get the proper safety maintenance procedures in effect.

If you mix the 4-20 mA with the solenoid 24 VDC, the larger the solenoid coil, typically the larger the risk of propagating noise on to the 4-20 mA circuits, but it will be a short burst of noise and if they occur only occasionally, you may not have any issues. However, I would recommend that the 4-20 mA and the 24 VDC solenoid signals be wired with individually shielded twisted pairs. This is important for the solenoid because it may be possible to propagate noise not only to the 4-20 mA signals upon solenoid operation but on to another solenoid circuit when a solenoid switches causing a solenoid misoperation. The shielded cable will also provide a improved electromagnetic interference profile for the 4-20 mA verses the 24 VDC solenoid signals and reduce the overall noise on your circuits that might provide a misoperation or even in some cases, it may help protect against damage to your connected devices due to other transients

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.
 
B

Bob Peterson

I won't argue with Bill about whether it is a good idea or not to have multiple junction boxes. But there is no US code that requires it, and it is very common not to do so. As he points out there are some good design issues that might make it worthwhile to do so, but these are design considerations and not legally required.

There is also some benefit to using separate boxes for voltages considered by NFPA70E to be lethal versus those that are not considered lethal that you would not gain just by separating the voltages inside the same box. But again, this is a design consideration and not a code issue.

--
Bob
http://ilbob.blogspot.com/
 
W
I certainly agree with Bob regarding it not being in the NEC (NFPA70). However, many people view the codes and standards as the "maximum" requirements (e.g. if you follow the code you are guaranteed safe) whereas they are really the minimum requirements and you are still responsible for a safe working and operating environment for your employees. Another consideration is what does the Authority Having Jurisdiction (AHJ) have to say about it and if there are any local codes that have to be considered.

If you have an accident where someone is shocked and killed because they were working on a 24 VDC 4-20 mA circuit and got on the 230 VAC by accident, you will not have much of a defense if the technical basis for having only one junction box was the code didn't require two boxes.

Also, if you put all the multi-voltage circuits in one box, your safety procedures for working in the box will be those of the highest voltage. When I worked in the plants as an electrical engineer, the break point for having to have a special electrical permit was >150 VAC to ground. 230 VAC may also affect the hot work permit requirements for working in the box.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.
 
A

anaalmansuri

Hello,

I have seen an enclosure box i.e marshaling cabinet located in a safe area with different voltages being located inside but on separate Terminal Strips (TB's). The potential 230v AC circuits were clearly covered with a plastic transparent cover and labeled with electric shock sign. Believe me it was not pleasant working in that environment!

To keep your life easy and secure you always mind safety regardless the code because the code tells you just the minimum requirement. So always remember when the bad happens all fingers will point to you!
 
B
these days the "break point" is 50V. The most common electrical safety standard (NFPA70E) requires special PPE (such as insulating gloves) when a worker is exposed to potentially lethal voltages (over 50V according to NFPA70E).

as part of the required (by OSHA) electrical safety plan, someone who might be working in a box has to be able to tell what voltage levels might be in the box so he can don the appropriate PPE required. This only deals with the electrocution hazard associated with electricity.

there is nothing inherently more unsafe with 230V than 115V. My understanding is more people are killed by 115V in the USA than any other voltage. Not real surprising given that 115V is the most common voltage people get exposed to.

--
Bob
http://ilbob.blogspot.com/
 
B
I have seen an enclosure box i.e marshaling cabinet located in a safe area with different voltages being located inside but on separate Terminal Strips (TB's). The potential 230v AC circuits were clearly covered with a plastic transparent cover and labeled with electric shock sign. Believe me it was not pleasant working in that environment!

To keep your life easy and secure you always mind safety regardless the code because the code tells you just the minimum requirement. So always remember when the bad happens all fingers will point to you!
==============
The finger will be pointed at you even if you go well beyond the bare minimum requirements. One of the problems with guarding terminals in the way you have described is that it is difficult to work with them for debugging. This often leads people to just remove them. Most terminals these days are "finger safe" anyway meaning it is harder to electrocute yourself.

Marking the different voltage levels is not a terrible idea but I am not sure that you gain as much safety wise as you might think. But between marking them and using finger safe terminals and segregating terminals by voltage level you have made it at least somewhat easier if not safer to work in the box.

My personal opinion is that people who are not competent enough to take the appropriate safety procedures when dealing with electricity should not be allowed to mess with electricity at all. That is OSHA's take on it as well ( a U.S. thing). If the guy working on the stuff in the box is unable to read the drawings and tell that there might be hazardous conditions there, his employer should not be allowing him to do this work.

--
Bob
http://ilbob.blogspot.com/
 
<i>I certainly agree with Bob regarding it not being in the NEC (NFPA70). ...</i>

I believe both you and Bob are understating the potential for the 24VDC and/or the 4-20mA power sources being Class 2. I'm not saying Class 2 is required or even commonplace, but I've seen many which are. Just because it is uncommon or people just don't bother to look, does not change the fact that it may well be a Class 2 source.

<b>And being powered by a Class 2 source requires separation from pretty much all other forms of powered circuits.</b>

For curiosities sake, how many power supplies shown on the following web page are Class 2? FWIW, that's a rhetorical question for I do not know for certain how many... but I do know some are and in that they are is not plainly obvious. Reminds of that deer in the headlight look you get from someone in the field when you point something out to them that is only obvious <i>when you look</i>.

http://www.solahevidutysales.com/power_supplies.htm
 
B
Actually being powered by a class 2 source does not make it a Class 2 circuit. You can still treat it as if it's a Class 1 circuit if you handle it appropriately. It's pretty rare to see Class 2 power supplies at least in my experience with a very few exceptions because Class 2 power supplies are limited to 100 VA which is only about four amps at 24 volts.

There are no real good reasons to use class two circuits in most industrial environments, with a few exceptions. Pretty much the only advantage to using Class 2 circuits is you don't have to use chapter 3 wiring methods but in most industrial environment you're probably going to use chapter 3 wiring methods anyway so why screw around with class 2 circuits.

As for separation it's true that class too circuits are usually required to be separated from other circuits. But that doesn't mean that they can't be in the same box as long as there in a separate part of the box or separated by a divider of some sort.
 
You can debate the disadvantages of Class 2 source all you want. I am not saying there is any advantage other than perhaps convenience (such as using a DIN rail mount power supply for a few circuits; BTW, 4A will power how many 4-20mA circuits?). What I am saying is, enumerating the disadvantages does not change the fact that many are used, and it may save some headaches if one looks before leaping to a false and possibly costly conclusion.
 
I would separate the AC from the DC, controls from power. But I would first engineer out the 230V by changing it to 24VDC with a relay at the equipment. Avoid that arc flash suit and hot work at all costs.
 
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