OT control power who says I gotta have 120 or less

leitmotif

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Nov 2004
Location
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OK some background
In the Navy just about all equipment used 440 (or 250DC for DC equipment) for control power. 440 was obtained from two of 3 phases.

I have also in civilian world seen 125 (and I believe 250 DC) for control power at Trojan Nuclear plant. But this was for switchgear control only not motors if I recall correctly.

Now I know the NEC excludes shipboard and utilities.

I also like the idea of low voltage ie 120 or less control power ESPECIALLY with ground referenced wiring. A ground on 220 or 440 control circuit could be enough to energize the contactor. That would be the SxxTS if you thought it was turned off.

BUT the question is this
Who says I gotta use 120 or less for control power?
I cant find it in NEC - would it be NFPA 70??
If I recall correct NFPA 70 is Electrical Worker Safety.
Is there not another NFPA "chapter" for motor control?

I'm doing this at home, and I do not have a set of NFPA at work much less home.

Could someone tell me the book and IF they got a few minutes give the chapter # and a quote where it says "thou SHALT"

Thanks
Dan Bentler
 
Sorry, but I am interested in your dilema....
You say that ships use 440 L-L for CONTROL POWER? Are they nuts?
WHY?
Use 24VDC! Only distribution and MCC should be 485,
 
I would guess unless you are a shipbuilder or Admiral, you should stay away from calling their electrical system nuts. I can think of lots of reasons why they wouldn't want the extra equipment needed to make 120VAC or 24VDC on a ship.

Back on topic, NFPA 79, Electrical Standard for Electrical Machinery, states:

9.1.2 Control Circuit Voltages.
9.1.2.1 AC Control Circuit Voltages. The ac voltage for control
circuits shall not exceed 120 volts, ac single phase.

Exception No. 1: Other voltages shall be permitted, where necessary,
for the operation of electronic, precision, static, or similar devices used
in the control circuit.

Exception No. 2: Any electromechanical magnetic device having an
inrush current exceeding 20 amperes at 120 volts shall be permitted to
be energized above control voltage through contactor or relay contacts.
The contactor or relay contacts shall break both sides of the circuit
powering the magnetic device. The relay coil shall be connected to the
control circuit.

9.1.2.2 DC Control Circuit Voltages. DC control voltage shall
be 250 volts or less.

Interestingly, I could find no such limit in UL508 but it could still be there, it's getting to be a big document.
 
leitmotif said:
OK some background
In the Navy just about all equipment used 440 (or 250DC for DC equipment) for control power. 440 was obtained from two of 3 phases.

I have also in civilian world seen 125 (and I believe 250 DC) for control power at Trojan Nuclear plant. But this was for switchgear control only not motors if I recall correctly.

Now I know the NEC excludes shipboard and utilities.

I also like the idea of low voltage ie 120 or less control power ESPECIALLY with ground referenced wiring. A ground on 220 or 440 control circuit could be enough to energize the contactor. That would be the SxxTS if you thought it was turned off.

BUT the question is this
Who says I gotta use 120 or less for control power?
I cant find it in NEC - would it be NFPA 70??
If I recall correct NFPA 70 is Electrical Worker Safety.
Is there not another NFPA "chapter" for motor control?

Thanks
Dan Bentler

Who says???

I have a copy of NFPA79 Electrical Standand for industrial Machinery and section 9.1.2.1 AC Control Circuit Voltages states:
"the ac voltage for control circuits shall not exceed 120 volts, ac single phase." There are 2 exceptions, one is for electronic precision or similar devices and the other is if there is more than 20 amps inrush at 120 VAC.
 
In several plant I have worked in the motor start/stop and interlock circuits are 480V. The 480V control power is picked up from the disconnect feeding the starter. Most of these are 30+ year old systems. If I am going to design a new system or modification I will use 120V as the control power. Although 480VAC control power works fine, I dont like it because of the higher "potential" (pun intended) for a flash if something gets grounded. Of course I you have a delta system you can ground one side and never know about it until you have the second ground...
 
Today, it's all about worker safety

Don't have a copy of it handy so I can't quote chapter and verse (think it's Section 130) but per NFPA 70e, you basically need to be less than 50V or provide sufficient protection to prevent shock/ electrocution/ etc. This protection means at a minimum you are working with rubber gloves with leather gloves over them to protect the rubber. You may also need other protective clothing from natural fiber clothing (e.g. no polyester blends) to a flash suit depending whether the the control voltage is mixed with power voltage that has a flash potential. We have this latter situation a lot in our older machines and it is ever time consuming to troubleshoot when all the power must be verified off to allow you to install probes.

Yes, you could do a lot of things differently five or so years ago before NFPA 70e but looking back, it was basically Russian Roulette: Statistically, if you do it long enough, you will eventually loose. Remove all potential of an accident occuring and it can not happen.
 
All my european experience was 24v DC control power.

Much nicer than having to mess with 120 V.

I dont see ANY reason to have 120 V control power on a new system apart from the usual 'weve always done it that way' BS.

I say get into modern times and use 24v DC.
 
With the advent of power supplies that take 3 phase 480 and make 24VDC, we've been able to eliminate the control transformer on several of our panels. While the cost of the power supply is slightly more, getting to take out the cost of the transformer, it's related protection devices and possibly even getting to downsize the box makes it an overall cost savings most times.

<shameless distributor plug alert on>

Phoenix Contact is one supplier of these power supplies:
http://www.phoenixcon.com/products/interface/powersupplies/

Note: I work for a distributor of Phoenix Contact Products.

<shameless distributor plug alert off>

Other manufacturers make similar devices.
 
SLC_Integrator said:
All my european experience was 24v DC control power.

Much nicer than having to mess with 120 V.

I dont see ANY reason to have 120 V control power on a new system apart from the usual 'weve always done it that way' BS.

I say get into modern times and use 24v DC.

If you have long runs in a bad enviornment there are plenty reasons to go to 120VAC. I too worked in Europe, and completly understand your thought process. It is different if you move to the heavy manufacturing world like woodproducts and certain chemical plants, concrete plants and the like.
 

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