Voltage Level ,JB

There are no rules to say you can't put it in one junction box. It depends on the signal levels, signal type, wire type, amount of noise on the power side etc. As long as you can take precautions to make sure the signals won't be affected by interference from the power side, one box is no problem.

Don't know where you are located, but in the USA all wiring in the box must have insulation rated to withstand the highest voltage present in the box. As for interference and noise, that is a different matter.Properly shielded and grounded signal wires should not present a problem.Since the power is DC it should not introduce any noise of it's own accord. When grounding signal wire shield, ground only one end of the shield.If you ground both ends,noise can be introduced instead of grounded out. Hope this helps HTRN

Hi all,

I think we would need to know the area classification before we could tell you that. I would also like to know if you are refering onlly to DC signals or do you have AC as well?

As a rule of thumb - keep your AC and DC seperate. I would also keep my DC voltages seperate. Keep your instrument signals apart from your power signals. Partly for noise but also for safety as the person working in the box might not be familiar with other electrical signals.

It is fine if you have your instrument signals together i.e. four 4-20mA (24VDc) signals and one 0-10VDc signal in the same box, for example as they are instrument signals.

What is not good is using your JB to marhsall 220VAc and instrument signals together.

Regards,

Craig

what kind of signals are you talking about? Thermocouple signals are mili-volt level and microvolt level and need very carefull attention to reduce noise. Many others fall into this category. Just because you have hosed up your design and run your signals where they pick up noise does not make your vendor FNG, in thuth it probably means that you have little experience with the signals and equipment in use and screwed it up. Let him help you out of the mess, and learn from what he is telling you.

T/C in FNG System , Super Info

Thanks

Hi all,

In the PetroChem environement we do the following (note it is a zoned area i.e. Intrinsic Safe):

1. IO cards to marshalling cabinets - dekabon single pair OS/IS (overall and individually screened) these are all 4-20mA 24VDC signals, mostly active loops, some will be passive depending on the instrument

2. Marshalling cabinets to field JB's - dekabon multi pair OS/IS (24 or 48 pair) again it will be 4-20 mA 24VDC

3. JB to instrument transmitter - dekabon single pair OS/IS 4-20 mADC

4. In the case of the instrument being a thermocouple I usually use a head mounted PUK depending on the location. This reduces the length of the compensating lead required. So it will still be JB to instrument single pair OS/IS 4-20 mA. The manufacturer will install the PUK in the TC head and connect it to the TC.

5. Where it is not feasable or possible to install the PUK in the thermocouple head, I install the PUK in the JB. So from step 3 we run dekabon OS/IS thermocouple compensarting lead from the PUK in the JB to the thermocouple itself. Also note that the JB will still go back to the marshalling cabinet via multipair. I might also have other normal 4-20mA signals in this same JB - this is not a problem.

An important point to bear in mind is the following: the screen should only be earthed on one side of the loop. This screen should go to a high quality earth. Having experienced huge problems with lightining I do the following:

1. Screen on transmitter side - not connected but covered in heat shrink.

2. Screen connected all the way back through JB - marshalling cabinet - IO card.

3. Screen on IO card side to HQ earth.

Please bear in mind the earth point that you use. Your whole DCS/UPS and instrument earth system should be the same. If not you will induce your own interference and create a huge problem to find.

Although as stated that we follow this procedure in petrochem (note no barriers as the card type will be IS rated otherwise insert a barrier between IO card and marshalling cabinet), I and fellow colleagues use this same principle with mining and similar environements.

As you are not working in an IS environment you are not worried about reactance and capacitance of the loop - but it is good practice to calculate the maximum length that you can run the total loop. In theory you should never get to this length as it is over 8000m for dekabon 1.0 mm² OS/IS cable.

Also keep in mind that nowhere in the above do I carry anything above 24VDC signals. The digital signals may be carried in the same JB and multipair as the analogue signals - but they are 24VDC. If you need 110/220 VAC - use a seperate PJB (power juction box) and these cable should be PVC/SWA/PVC. Also keep the cable seperated in their routing. A good rule of thumb - if the instrument signal cable and the power cable cross each other perpendicular i.e. at 90deg seperation should be at least 150mm. If the instrument signal cable and the power cable run together in a parallel fashion the seperation should be at least 500mm. (this is a SANS standard for South Africa - but should have an IEC/IEEE equivalent)

I have found in practice that the above spacing also works very well when working with bus systems in substations and the like. i.e. Profibus to VSD's etc.

Regards

Craig

hi

I want to ask can i combine digital and analog signal in a single muticore cable? secondly can i combine input and output signals in a single muticore cable?

My problem is that i have 2 analog inputs 2 analog outputs and 3 digital inputs at same location? what will you suggest me?

Hi there,

As long as they are instrument signals i.e. 4-20mA 24VDC and or 0-10 VDC etc for the analogues and your digital signals are 24VDC you should not have any problems using a multi pair cable. You should note that you should use individualy and overall screend cables though.

If you have VAC signals - run them seperate and keep them spaced.

Regards,

Craig