Blower wiring - Is split 220 kosher?

[jsa]

[jsa]

[Jesse Lackman1]

I was just checking our house panels to see if they are isolated ground.

The house panels could be isolated ground but are not.

The neutral in the coop box outside is grounded. This is what is confusing to me. If the neutral is bonded to ground in the coop box what is the advantage of an "isolated"ground in the house power panels?

[Tony]

The idea behind the green ground wire is that if the insulation fails within a device, leakage current may cause a metal enclosure or cover to become dangerously "alive".

Current can then flow through your body to ground if you touch the device. You may feel anything from a mild tingle to a very lethal jolt.

The ground wire provides a much more direct path back to earth than your body, diverting any dangerous current safely away. It is there for your protection only, it plays no role in supplying power.

The neutral wire is usually also connected to ground at the main power board, but has a very different purpose. It is the return path for the main current powering the device.

The neutral wire absolutely must not be used as a safety ground. This is very dangerous, because if the neutral wire were to fail or become disconnected at the power board end, the equipment would then become "alive".

[jsa]

[thomasvaught-1]

If you have an electrical panel in your house and have a "Sub Panel" in your detached garage, then
the main panel would be your "house panel" and the "sub panel" would be in the detached garage.
The "sub panel" would have a green ground wire going from the ground buss bar to a copper rod driven into the ground close to the panel.

Tom V.

[Jesse Lackman1]

[jsa]

[Jesse Lackman1]

Well in my case the power company installed a pad transformer and disconnect box with a meter which is set up with a standby generator switch. This box is about 8 meters from the house. It has an earth ground connected to the neutral from the pad transformer. There is underground wiring (three conductors) from this box to a 250 amp breaker box on the outside of the house. There is an earth ground rod connected to the neutral in this box. Four wires go from the 250 amp breaker to the main house panel. There is a ground to earth ground rod out of the main house panel. The house sub panel is right next to the house main panel and is wired across to the main house panel with four wires, no seperate ground on this box.

The shop is fed off of the power company box, three wires to the shop panel and this panel has its own ground to earth ground rod.

The individual 120V circuits in the house are three wire, hot, neutral and ground. The 240V circuits are four wire, 2 hots, neutral, and ground. All the grounds are brought back to a ground bar (a bar with a bunch of holes and set screws for holding the wires) in the breaker panel. The neutrals come back to a neutral bar, the hots go to the breakers. The earth ground rods are connected to the ground bars. This would be similar to your MEN.

Um ... What was the question? :p

[Dave W]

[Dave W]

[Jesse Lackman1]

Thanks everyone, it finally makes sense.

In other words all sub panels must have isolated ground bars, unless they have their own ground to earth ground rod.

This would include a flowbench "sub panel" - a flow bench breaker box should be isolated ground unless it has it's own specific ground to earth ground rod.

~

I've been thinking about wiring 120V motor pairs with 240V and the problem of killing a good motor if one fails. I'm not understanding why this would happen. Evidently SF benches are wired this way - do they use circuit breakers like a house panel for motor pairs? Is there something unique in the way an SF bench is wired that enables this problem? Why isn't a circuit breaker opening when a motor fails??

I can see the failure of one motor of a motor pair not tripping a 240V breaker. The only way extra voltage could get to the running motor is 1) if the 240V breaker did not trip, 2)the failed motor created a flow path *through the neutral* to the running motor. If a neutral suddenly has 120V in it from the other pole (enough to fail the good motor of a motor pair) surely other bad things will happen especially if the wiring diagram at the top of the page is used where the neutrals are tied together all over the place. There would be have to be more motors/contactor coils/etc. getting an overvoltage than just the motor paired to the one that failed.

I'm wondering if this could be prevented by;

1) Not using double pole breakers to feed the motor pairs. If you use two single 120V breakers tied to seperate 120V poles and a seperate neutral for each motor pair, one motor failing would trip one breaker and the running motor would simply stay at 120V.

2) Run seperate feeds and neutrals to the contactor coils.

I bought a Siemens 125A mail lug panel with 8-16 circuit capacity. The breakers are paired 120V 15 amp breakers.

Bench power for contactor coils, lights, etc. will be fed off of the top two breakers on each side. That sets up the lower breaker pairs on opposite lugs or 120V poles.


I'll be running all motor contactor coil power though the top contactors in this picture; The main power switch for the bench is a two pole Carling rocker switch/breaker, one pole will close in the top contactors (which feed the motor contactor coils) and the other pole the bench power lighting, motor contactor coil power etc. The first four motors will be on one Carling rocker switch/breaker, the next six motors will be switched in pairs.

[Tony]

Jesse, the problem with wiring two motors in series directly across 220v is the manner in which they fail.

Something like a light bulb just goes *pop* and always fails open circuit. So it is entirely practical to wire light bulbs in series and supply them from a higher voltage. If one fails, they all go out, with no damage to the remaining bulbs.

The type of motors we are discussing very rarely go open circuit. It can happen, but it would be quite unusual. Much more likely the motors will overheat, burn out, and fail shorted.

Typically, either the field winding or the commutator starts to burn, and the insulation progressively fails. The motor runs hotter and hotter, until it finally either goes bang, bursts into flames, of blows a fuse or trips a circuit breaker.

If two motors are connected in series the motor that is self destructing tries to draw more current, and this increases the voltage across the good motor.

The good motor starts to over speed and run hot because it may be receiving far higher than 110 volts, anything up to the full 220 volts is possible. So the good motor is usually either damaged or destroyed as well.

By connecting the neutral wire to the center point, each motor can only ever receive a maximum of 110 volts no matter what any other motor is doing. A single motor can still fail, but it will not directly kill any of the others.

[thomasvaught-1]

X2 on Tony's comments!!!

Tom V.

[Jesse Lackman1]

"By connecting the neutral wire to the center point,"

What's the "center point"? The center point (white wires on drawing referenced in top post) between two motors?

ie

good = seperate pole breakers to motor pair blacks - motor pair whites connected and brought back to neutral in breaker box

bad = seperate pole breakers red and black (240V) to motor pair blacks - motor pair whites connected but do not return to breaker box neutral