Reverse Polarity Effects On Electronic Devices Question

[streets]

Ok Guys/Gals,
I have searched the archieves and determined its best to ask the experts on this one.
Simply reversing AC+ and a Neutral in a toaster or light socket creates a shock hazard, but the device might still work.

I have a traffic signal utility cabinet that has the following electronic devices in it & would like to know what effects reverse polarity might have on them: the circuitry starts with a 120vac/ 60 amp surge suppressor, Mercury contact switch, 24vdc Bus Interface Unit (BIU,) a series of 12 load switches that convert 12vdc to 120vac, (4) 120vac field transfer relays, 24vdc microprocessor, 12vdc/24vdc power supply with a huge capacitor. That's the short list.
Besides unstable operation, is there a way to tell this unit has been backfed?

I should add that Neutral and a Green (grd) circuits coming from the power meter (utility) service are bound all the way around a traffic signal intersection (all 4 corners.)
This particulay utility cabinet was installed with the AC+ connected to the Neutral from the meter, and the meter Neutral connected to the AC+. The ground wire was connected correctly.
Any insight on past expeience is appreciated.
Dave

 

[LawnGuyLandSparky]

Ok Guys/Gals,
I have searched the archieves and determined its best to ask the experts on this one.
Simply reversing AC+ and a Neutral in a toaster or light socket creates a shock hazard, but the device might still work.

I have a traffic signal utility cabinet that has the following electronic devices in it & would like to know what effects reverse polarity might have on them: the circuitry starts with a 120vac/ 60 amp surge suppressor, Mercury contact switch,

Neither of these could care less about polarity.

24vdc Bus Interface Unit (BIU,)

Which is a rectifier, also doesn't care about polarity.

a series of 12 load switches that convert 12vdc to 120vac,

The load packs are nothing more than electronic relays. Overgrown transistors, really. Polarity of their line voltage i/o is irrelevant.

(4) 120vac field transfer relays,

Typical 4p, double throw relays to flash in manual mode. Doesn't care about polarity.

24vdc microprocessor, 12vdc/24vdc power supply with a huge capacitor.

You left out the conflict moniter. But the list of DC components above is of course powered by the DC from your power pack, and couldn't suffer from a reverse polarity from the A/C power supply.

That's the short list.
Besides unstable operation, is there a way to tell this unit has been backfed?

I should add that Neutral and a Green (grd) circuits coming from the power meter (utility) service are bound all the way around a traffic signal intersection (all 4 corners.)

This is typical DOT standard around the country, which many are begining to realize wasn't a good idea. And it isn't. Bonding the neutrals on the poles will in theory provide a ground, but it ends up creating more problems when something goes wrong upstream. BTW are you sure about the utility ground on all 4 corners?

This particulay utility cabinet was installed with the AC+ connected to the Neutral from the meter, and the meter Neutral connected to the AC+. The ground wire was connected correctly.
Any insight on past expeience is appreciated.
Dave

This should have resulted in a direct short. Does this facility have an isolated power supply by any chance?

 

[petersonra]

nothing that uses AC cares one whit what the polarity is in virtually all cases.

dry contacts (relays, contactors) normally do not care if they do not have magnetic blowouts. Units with magnetic blowouts may not work correctly if the polarity is reversed.

some dc contactors require you to use multiple poles in series to interrupt some (usually higher voltage) loads, and polarity normally does matter in these cases.

devices such as transistors, scrs, LEDs, and diodes do care. bridge rectifiers normally do not, so sometimes you see a bridge in front of a polarity sensitive device so it is no longer polarity sensitive.

dc motors run back wards if the polity is reversed.

resistive loads such as heaters and incandescent bulbs do not care what the polarity is.

 

[ELA]

Interesting.
How was it discovered that the feed lines were reversed? How long had it been powered up in that fashion?

It does seem that if the service is bonded, and there are also downstream bonds at each pole that the main breaker should have tripped?

I can think of some very abscure instances on some not so well designed equipment where it could make a difference but nothing is jumping out at me.
I have worked on a couple of pieces of older equipment that "sensed" the Hot lead with respect to PE that would fail to work under these conditions. Thats a bad design though.

 

[masterelect1]

Plc

Plc

Commenting solely on the PLC, if the DC is reversed going to the I/O racks the PLC could be damaged as I/O is desinated sinking ( +dc out, - dc return)
or sourcing ( -dc out, +dc return) I could have that backwards. I've been away from PLC's for 2 years but the I/O is critically sensative to polarity

I/O = input/output (I'm sure you knew that)

John

 

[ronaldrc]

I don't think you mentioned if the 120 VAC feed had a switch on it or not?

But if it does and it is just single pole switching the hot side of the solid state cir. then all the exposed wiring will remain hot to ground even with the switch off.

If the solid state board is isolated from the ground other than what I mentioned it should be ok. but I would reverse it anyway. Even if it is a double pole master switch. I am assuming the outputs are of a general purpose relay with hard contacts and neither side grounded?

 

[nakulak]

worst case scenario (I can think of): if the line was incorrectly wired to the load side neutral (but the line side neutral is correctly bonded to ground at the source), it could be possible that a life-threatening hazard exists at the light poles (or exposed metal parts) due to exposed metal parts being energized. It is possible that there is enough resistance at the grounding ring to prevent the breaker/fuse from blowing, but still providing for energized parts which will kill someone as soon as they provide a good path to ground (rainstorm and leaning on post, etc)

 

[mxslick]

<snip>nothing that uses AC cares one whit what the polarity is in virtually all cases.......

Not quite. In the cinema biz we had a film platter transport known as the Autowind 2 (or more appropriately, the Autoswine 2) which was very sensitive to the correct connection of the AC line.

If line and Neutral were reversed, all three motor control modules would let out the magic smoke, violently.

It was a control design with the secondary center tap of the 24vac control transformer connected directly to the neutral. The driver circuitry for the motor took incoming AC, passed thru a single diode and fed to the DC drive motor. One of the components in that driver circuit referenced the AC line and the control power.

And also as a side note, for the sake of discussion, the term "polarity" as used here isn't quite correct, as AC in the strictest sense doesn't have "polarity".

I think the terminology should be "reversal of line and neutral."