ECN Forum Forums Code Related Discussion NEC & other Code issues 480V on push button controls.

As long as every part in the control ckt is rated for what ever voltage ( and current) that is present, its code. It makes no difference as to phase to phase or phase to ground, everything must be rated for the highest voltage found in the ckt. Over current protection must be in place where required. I agree with most that low voltage (24v or 120v) control is much preferred. Robert

The mine and mill that I work in were built in the early 1960's. All of the original controls are 480vac delta. Quite alot of them don't have and EGC either, mostly solid wire. Hard part is explaining how the voltage is present on hot and reutrn side of the cicuit with the switch open.

MSHA has it's own set of rules when it comes to electricty, otherwise how could you drag a 200 foot long cable behind a machine running on 480vac? And allow the operator to move it into and out of hangers when it's energized?

PS MSHA electrical is 30 CFR Part 57 Subpart K and 30 CR Part 18

You are measuring the line voltage between two legs minus the voltage drop from the load that the push button controls, likly a coil.

Look at it as a series circuit. The open contact and presume (never assume) for this dissussion is a very resistive load. It will be in series with the coil. Although the coil resistance is high, it will be much lower then the open contact. In a series circuit, the voltage of the circuit will be the sum of voltage of each load within the circuit. In addition the voltage at each load will be inversly propotional to the resistance of each load. The higher the resistance is of a load, the lower the voltage for the load will be.

If we assign a resistance value to each load, R1 being 10 Gig-ohms (open contact) and R2 was 50,000 ohms (coil), the resistance of R2 makes up only .0000005% of the total resistance of the circuit so the voltage will be 99.9999995% of the total voltage. Being that the line voltage is so high the voltage drop is less across R2. Although it appears that the voltage is the same, when the contact is open, you are actually measuring is the line voltage minus the voltage drop.

A digital voltmeter will likely pick up the voltage but not a analog meter due to its high resistance.

No, actually its because L1 is phase one of the delta and L2 is phase two of the gelta and either one to ground will give you voltage less than 480 if the switch is open

How is this delta grounded. That will be the deciding factor.

Delta ground is derived from the primary side of the xfmr at each substation in the plant. I know how it works and my boss found out the hard way when he got zapped by the L2 on a control circuit.

Actually a funny story, him and two other guys were troubleshooting a starter, one had a the L2 in his hand (?), leaned on another guy and then he leaned on the foreman who was in contact with the side of the box. Don't know how they did it, but needless to say they all got a buzz.

The secondary usually has to be grounded somewhere too, with a few limited exceptions. Where you ground this can make a lot of difference what the phase to ground will be.
I think this is why they use 3p wye a lot on 480. It keeps your L/G down to 277.
If you corner ground a delta you get 480-480-0 but if you centertapped it you get 240-240 and a 415v "wild leg".

You need to look in NFPA79 the standard for industrial machines and in IEEE/ANSI C47xxx the standard for switchgear.
These codes all but prevent any voltage to the door >240V.

When you buy a 480 volt combination starter it comes standard with 480 for the stop/start buttons. They use the supply voltage for local control. You have to order and pay extra for a 24 or 120 volt coil and subsequent wiring change on the starter. 480 on PB's are as common as butter on bread.