120V LIMIT FOR CONTROL 2

[BlackJackJacques]

I can't seem to find a solid basis for the notion that I have been applying for as long as I can remember with regard to limiting the control voltage for hand-actuated devices to 120V. I am looking for an NFPA, OSHA, etc cite. I did find a reference in IEEE 45 that limits control voltages in devices running external of a controller to 120V, but I expected to see this constraint everywhere. Can anyone help? Thanks in advance.

 

[dpc]

In the US, 120 V is not a limit. A bare bones NEMA motor control center will have 480 V (or 600 V) control power. Or at least that was case in the past. The standard NEMA heavy-duty pushbuttons, switches, etc are rated for up to 600 V. The NEC treats anything below 1000 V basically the same.

Shipboard standards may be different, I don't know.

Obviously, based on what we know about arc flash hazards and shock hazards, 480 V or 600 V control circuitry is not a good practice due to safety concerns.

 

[BlackJackJacques]

dpc - Thanks. This requirement is almost primordial to my basis and it is bugging the heck out of me that I can't find it in more places. I have been treating every hand control using this guideline in both ship and land applications, but aside from what I found in IEEE 45 (and even that is a weak "should"), it is flaccidly referred to here and there.

 

[jraef]

NFPA 79, Electrical Standard for Industrial Machinery.

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 at line voltage through contactor or relay contacts. The
contactor or relay contacts shall break both sides of the line voltage
circuit to the magnetic device. The relay coil shall be connected to the
control circuit.

NFPA 79 is what supplanted the old "JIC" (Joint Industry Council) standards that were, for the most part, related to the Automotive industry back when they dominated all things industrially related. "The standard shall apply to the electrical/electronic equipment, apparatus, or systems of industrial machines operating from a nominal voltage of 600 volts or less, and commencing at the point of connection of the supply to the electrical equipment to the machine."

Now, whether or not YOUR equipment meets the criteria for needing to follow NFPA 79 standards or not is a nebulous issue, often contended, but only by people who want to avoid any added costs. Following NFPA 79 is basically "voluntary" in that there is no enforcement or policing body associated with it. But it is considered a "common practice" and if something bad happens where lawyers get involved, you can spend a LOT more money defending your decision NOT to follow it that you would spend just following it.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[BlackJackJacques]

Jraef - God bless you. I fully concur with your assessment regarding it being far easier to defend oneself using a widely-accepted standard of care, particularly in the adversarial three-ring circus called a court of law. In our case NFPA 70 is incorporated by reference into the governing federal statute, and NFPA 79 is a second-tier reference under NPFA 70, so I can make it stick if they decide to start throwing chairs. Thanks again


Black

 

[jraef]

To acknowledge what dpc had said though, when I started out as an electrician at a steel mill in the late 70s, almost all of the starters we had were 480V controls, even the pilot lights on the door. It used to be a lot more commonplace than 120V control with a control power transformer, likely because you saved the cost of that transformer. That started changing in the 1980s as safety concerns arose. In fact to this day, older JIC standard wiring diagrams for motor starters always showed the OL trip contacts on the right hand side of the contactor coil, prompting people who see it that way now to ask "Why do they switch the neutral?". In truth back then, that was not a "neutrel" rail on the ladder diagram, that was just the other side (L2) of the line voltage control circuit.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden