Running High voltage with Low voltage

Quick question for you guys.

Today I was building a control enclosure.

I have 600vac, 120vac and 24vdc inside this enclosure.

It's always been a pet peeve of mine to never run 120 vac or 24vdc parallel to a 600vac conductor. I might intersect here and their but never run in parallel.

An engineer wanted me to run a 120vac wire inside the same wire duct has my 600vac.

I told him even tho both insulation's are rated for 600v it is bad practice to do this has their is a possibility of having dirty signals.

I am also certain I have read somewhere you need minimum 1" air gap or some kind of isolation barrier when doing this.

Any thoughts?

We run 600 and 120 all the time in the same pipes. I agree with you on the 24V separation. but as for the rest I don't generally see a problem. The exception would be VFD signals mixed in, those should be isolated.

As for the 1" air gap, as long as everything has 600V insulation, I can't see why you'd need it and I don't believe it's in the code book.

I have often run 400v AC & 24v DC wiring in the same trunking, with no problems whatsoever.

As long as any signals that may be affected by interferance, such a 4-20mA or RTD signals are run in screened cable there is no problem.

The only exception to this, if you are dealing with high current circuits, such as main cables to large motors, or generators.

Then I am careful in my cable routing.

An engineer wanted me to run a 120vac wire inside the same wire duct has my 600vac.

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I am an engineer and I want you to do that too. The type of 120 VAC wires can make a difference. If they are 120 VAC signal wires, then there could be problems with induced voltage causing spurious signals. If they are 120 VAC power supply wires, then probably not.

Marc_U said:

I have often run 400v AC & 24v DC wiring in the same trunking, with no problems whatsoever.

As long as any signals that may be affected by interferance, such a 4-20mA or RTD signals are run in screened cable there is no problem.

The only exception to this, if you are dealing with high current circuits, such as main cables to large motors, or generators.

Then I am careful in my cable routing.

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The rule we follow is to segregate "Supply" from "Signal" cables. The only reason for this is the noise that might be induced on our signals. Other that that there is no reason you cant do this.

As a rule we try and segregate the High voltage 600VAC from the Low voltage 12/24VDC within enclosures. i think it is good practice. The big culprit for noise i have found it the past are the VFD's, which did not have screened motor cables.

Rheinhardt

Common sense rules. Certain devices are more sensitive than others. How long are the runs? If you have a foot of parallellism between some 24v signals and some 480v signals, then there probably is not enough coupling there to make much difference. Just don't tie wrap them together. Segregation should be done as far as practically possible. The "practically" part is the key. Like everything else you can take it to an extreme and waste a lot of time, expense, and energy guarding against something that is extrememely trivial. I commonly run my low voltage (analog and 24VDC) together and my high voltage (115vac to 480vac) together. Sometimes if the 115vac are PLC inputs, I may separate those from high current power wires.

I had an engineer one time specify things to such an absurd degree that it probably literally tripled the install cost of the job. ALL signals had to be separated. Each encoder cable had to be run in its own conduit. 4-20mA signals could not be run in the same trough as 24VDC. We had a valve bank about 60 feet away, and he made us run separate common wires for every single valve on the bank. He performed an analysis on the grounding in the building and had the difference in Earth potential mapped in grid like fashion. I believe we ended up installing a few more ground rods here and there to satisfy his "concerns". When I questioned the guy on this he would start talking about solar flares and how the whole Northeast power grid went down a few decades ago. The machine was making material for diapers, I think we would have all survived.

^ You can never be too sure about them solar flares.

I agree with Lancie1, but I would add to just double check end of line voltage to make sure it doesn't actually increase or drop end of line voltage beyond acceptable ranges.

Most PLC inputs are current sensing, not voltage sensing and induced voltage will not cause an input to come on, or cause an output device (coils, lights) to operate. Analog signal wiring and communication wiring does need seperation as noise on those can cause issues.

Most PLC inputs are current sensing, not voltage sensing and induced voltage will not cause an input to come on, or cause an output device (coils, lights) to operate.

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True, but Mike did not say if a PLC is even used in these circuits. I do know from experience that 120-volt signal and indicator lamps can get enough induced voltage from parallel high-voltage cables to glow dimly even when they are supposed to be OFF. That can confuse operators and cause many questions. It is best to avoid running 600 volt and higher power cables in with your 120 volt signal cables (if there is still anyone out there using 120 volt controls).

Lancie1 said:

(if there is still anyone out there using 120 volt controls).

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Sometimes, in environments where the extra arcing helps keep the contacts clean. Still find it better for larger inductive loads like large valves or huge contactors/starters.

It's been common practice to run the 120 Vac control conductors in the same conduit as the 600Vac power conductors in motor circuits for years. When we switched to cable, composite cables with both power and control conductors in a common cable were very common.

A lot of this has changed since PLCs became common in motor control.

I agree with you on keeping the 600 Vac away from 24V control circuitry. I hate to see power in the same enclosure as a PLC. The whole arc flash issue has reallty brought this into focus.

I had responsibility for 36 tire assembly machines with 4 and six inch square ducting in a large trapezoid shape all through the two sided full stage beast about 7 feet from the floor. It was stuffed full of DH+, 480vac power, serial cables, analog, mostly though 120vac control to cam switches and pushbutton panels, all 70s stuff. The worst noise was from the old DC drives, but AC drive noise was rare, we upgrade Fincor AC drives to 1336 Impact drives with Reliance motors and econders full vector, common off the shelf parts, and used shielded cable.

Once I was troubleshooting one of hte conversions after start up it was hanging waiting on a DC voltage pressure signal. It was scaled to 0.1psi, and the signal seemed good...oscope revealed the tiny threadlike 400v noise spikes from that new drive, so we opened the panel and the shield was stripped and heatshrikend, laying loose, they forgot to land it. The electrician said, can I land this while he's running, I said sure it's just a shield, right, its terminated at the motor right? Yes, he did that one himself. There was a long blue spark before he sunk it in the block, and he looked at me, like , you m*&%$@!...But then my pressure problem was gone and the noise spike gone from the meter...We upgraded that machine in 36 hours tire to tire, from ISSC 8bit racks to a shiny spankin' new PLC5 with a Panelmate in 36 hours. There were like 14 different mechanical changes and switching from cam switches to resolvers.

I tried to go back and remove all the abandoned wire one down Sunday, and gave up after 9 hours back and forth. I got most of it out to the center and #1 section... You literally could not poke a 1/4" allen wrench in with the old wire. They filled it over 95%, over 30 years of mods and repairs, and we had to zip to the covers on...

First lets get something straight. The definition of high voltage is > 1000V AC (rms) or 1500 V DC. Low voltage is 50 - 1000 V AC (rms) or 120 - 1500 V DC. Extra-Low Voltage is < 50 V AC (rms) or < 120 V DC. It's a common misconception that mains voltage is 'High' and not helped by warning labels decrying DANGER HIGH VOLTAGE for anything with electricity.

So, anyway, the OP is really asking what people think about running power supply type voltage in the same duct as control voltages. I don't have a problem with it and I would suggest it is damn near impossible to segregate the two. For a control voltage you will need a power supply which will have to be fed with a supply voltage - try to keep the two apart in a real world situation. (I'm calling anything above ELV a supply voltage for want of a better wording).

The biggest issue is noise on a control or signal/analog circuit from a supply voltage. I find this can be negated with proper earthing and shielding. It doesn't really matter if the cabling is all bunched together or not.

One of my pet peeves at the moment is consultants insisting that 230V voltages must be kept separate from control voltages by at least 300mm. A classic case of book smarts versus real world applications and experience.

Do feel free to abuse me for not remembering all my university learning through the drunken haze. I'm just an electrician who does what the over qualified consultant engineers tell me to do - even if I think they're talking out of their a$$.

Winklehoffen said:

Do feel free to abuse me for not remembering all my university learning through the drunken haze. I'm just an electrician who does what the over qualified consultant engineers tell me to do - even if I think they're talking out of their a$$.

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Why would we do that, after all we have had to probably deal with such idiotic consultants.

One of our customers insists on separate trunking for AC & DC wiring, and then screams blue murder at the cost of the panels, because the resulting panels have to be 50% bigger than they need to be to accomadate the extra trunking.

They did try to insist (fairly sensibly) that there should only be DC (24v) in Common Control section, where the PLC is.
However, when this meant that the Common Grid Voltmeter & Frequency, would be mounted on one Generator panel door, and thus making it different from its neighbouring Generator panel, they instructed us to mount the meters on the Common Control door, invalidating the whole point of the "DC only" rule.


This same customer tried to insist that anything over 600v AC was high voltage - We were installing 690v AC Hydro-generators - and as such tried to force us to retro-fit all the protection, etc that is normally required for H.V. equipment, such as Earth Switches, etc, but they were unwilling to pay for the extra cost.

I think we won that fight, as their latest spec, now acknowledges that High Voltage is 1kV or higher.

Marc_U said:

...This same customer tried to insist that anything over 600v AC was high voltage -...

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This customer may have some experience with US codes. The NEC Handbook uses 600VAC,nominal as a dividing line for its rules. For Example 300.5(C)(2) "...Conductors of circuits rated over 600 volts, nominal, shall not occupy the same equipment wiring enclosure, cable, or raceway with conductors of circuits rated 600volts, nominal, or less ..." Elsewhere in the code, circuits rated between 600V and 22kV are grouped together.

I can see why they thought >600V is high voltage.

Winklehoffen's definitions of high voltage,low voltage, etc are interesting to me. I hear circuit board designer talk of 12VDC as high voltage because their circuits run on 3.3V. When I am unfamiliar with some one I like to start with a definition of terms. "What do you mean by high voltage?" is good question. Guys in the lab have a whole different idea compared to the shop floor as to what is a large motor. (10HP is big in the lab but a wee baby out in the shop.)