Electrical – Is it safe to touch a running 1 hp electric motor
acelectricityinsulationmotor
This question is about touching the housing of a generic AC electric motor like this one:
…while it's running, without considering any type of insulation, such as wooden floor, rubber gloves or boots, etc. Thanks.
Best Answer
The motor shown is has a totally enclosed fan cooled (TEFC) housing. The cooling fan is driven by the motor shaft and is covered by the housing around the motor on the end opposite the driveshaft end. For a motor of that size and type, it would be very unusual for the voltage to exceed 600 volts. The windings are well insulated from the housing. A properly installed motor would have the housing grounded by an equipment grounding conductor. For the motor to be unsafe to touch, it would need to be both improperly installed and not in good condition. The answer would be the same for a large home appliance except that home appliances are not connected to voltages above 240 volts.
It boils down to current distribution, but let's start from the beginning.
In both cases you form a circuit with several elements in series. Some of them have excellent conductivity, like copper wires. You are of meager conductivity, but your shoes and ceramic tiles have even worse conductivity, they are dielectrics. As the dielectrics have contact to or are in between conducting elements in a geometric relationship they form capacitors. This circuit consisting of capacitors and resistors permit AC current to flow. Aside from some other effects the current should be the same order of magnitude in both configurations.
What's the difference now between both cases? When touching the copper wire you form a conductive connection with a very small area for the carriers to pass. As two conductors connect, there can't be a strong electric field either. Due to the small area the current density maximises and you can sense it. n.b.: you can't sense it at your feet, can you?
The other case differs as you touch a dielectric. This dielectric differs from the surrounding air but not very much to change the electric field dramatically. Hence the electric field vectors connect to your skin perpendicular and with merely unchanged field strength. At your skin the electrical surface charge influenced by the field causes a current, but this time distributed over the surface of your hand. You can't sense it.
The wire gauge required by the NEC is determined by the circuit breaker protecting the branch. In the case of a short, even in a low current branch, the wire conductors need to be able to handle the momentary over-current of the short. If the short is not "perfect", the breaker will happily supply 60 Amps+ into it for an indefinite period of time. Not a good situation if the conductors are sized to carry 2.5 amps (presumably #14 wire, the smallest permissible gauge for branch circuits). For safety all branches of the circuit are assumed in the NEC regs to be able to carry the amperage dictated by the circuit breaker serving that branch.
By the way, if you have "long" cable runs to the high amperage motor you may find that the motor is starved for starting current due to the round-trip resistance of the conductors. This resistance may create a voltage drop in the wires sufficient to prevent the motor from starting, or cause it to start slowly or erratically. In your specific case you have that 25 Amp heater complicating matters. If the heater is also on the same long branch of the circuit, it will create its own voltage drop in the wiring which will affect the voltage available to the motor when it starts. In these cases you will need to use a heavier gauge cable than is dictated by the normal ratings of the motor.
Best Answer
The motor shown is has a totally enclosed fan cooled (TEFC) housing. The cooling fan is driven by the motor shaft and is covered by the housing around the motor on the end opposite the driveshaft end. For a motor of that size and type, it would be very unusual for the voltage to exceed 600 volts. The windings are well insulated from the housing. A properly installed motor would have the housing grounded by an equipment grounding conductor. For the motor to be unsafe to touch, it would need to be both improperly installed and not in good condition. The answer would be the same for a large home appliance except that home appliances are not connected to voltages above 240 volts.