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Earlier today I replaced my doorbell transformer in preparation to install video doorbells. The old transformer was connected outside the main panel, and only had 2 wires. One to hot, and the other to the neutral/ground bus. Of course, the new transformer has 3 wires (black, white, green). My first attempt to install, I connected the black wire to hot, and both the green and white to the neutral/ground bus. Turned on the breaker and nothing - no voltage on the meter. However, when I removed the green wire from the neutral/ground bus (leaving white and black where they were), all worked fine - so I just capped the ground wire and used electrical tape to stick it to the inside of the panel box. Any idea why the transformer did not work with the ground wire connected to the bus? Something special about connecting at the panel? Is the setup unsafe?

Thanks

Happy New Year!

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    Is the actual transformer screwed to, attached to the main panel? – JACK Dec 31 '20 at 22:59
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    @Matt -- is there continuity from the transformer case to the green ground wire? – ThreePhaseEel Dec 31 '20 at 23:50
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    @Matt -- before you call the transformer manufacturer, can you set your meter to "continuity" i.e. the setting where it beeps if you touch the probes to each other (or the lowest ohms setting, if your meter lacks a continuity setting), then, with the breaker powering the transformer off, measure from the green wire on the transformer to exposed metal on the transformer case? – ThreePhaseEel Jan 1 at 3:00
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    @Matt -- that is very curious indeed then, see if you can get a reading from the green wire to the grounding bar on the panel (via the loadcenter cabinet), then – ThreePhaseEel Jan 1 at 3:31
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    I exchanged the transformer for a new one and it worked with ground and neutral in the panel bus bar. Must have been a bad one before! Thanks all. – Matt Jan 1 at 18:53
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Follow the Instructions

The instructions say:

For metal junction boxes, it is required to connect the green ground wire from the transformer to the metal junction box or clamp it to the metal conduit.

If it doesn't work when connected properly then either you have something very strange going on in your panel or the device is defective.

The fact that the instructions actually say this, rather than "nothing" or "the green wire is only required if you are not installing the transformer on a grounded metal junction box", means that you need to do it for a proper installation. End. Of. Story.


Ground != Neutral

...except when it is :-)

In a perfect world (electrically speaking), ground would only ever be used for two types of current: device/wiring fault and external current (e.g., static discharge or lightning). In order to make everything work properly, ground is bonded to neutral at one place, typically the main panel.

For a bunch of historical reasons (e.g., separate ground wires have not always been a thing), there are a bunch of situations where there used to be, and in some cases still are, ground and neutral together at the device.

However, there is no requirement that ground and neutral be the same and be connected in a device. But there is, generally, a requirement that each device have a ground connection. (There are exceptions - double-insulated tools, simple devices such as wall warts or lights, etc.) With many devices, the case or exposed metal parts of a device will be connected to neutral, whether or not there is a separate ground wire. However, it is quite possible to build a device which has ground to the case (for safety) but which does not connect that ground to neutral because doing so would make the device not work properly. A transformer is a simple example of such a device.

End result: If you have separate ground and neutral connections (screw terminals or wires) on a device then you do not connect them both to neutral unless the directions tell you to do so. Doing so when not appropriate may cause the device to not work (no harm done) but may be deadly.

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ALWAYS hook up safety ground first.

Nothing is wrong with ground. Ground is a safety net. Ground catches faults. That's what it does.

The problem is that if your device DOES have a fault, hooking up ground LAST causes you to observe a condition where the device works (but it is dangerous because it's faulting and it's not grounded, but you don't know that). Then you hook up ground, and the grounding system catches the fault just like it's supposed to. But this also means the device doesn't work.

You're now barking up the wrong tree troubleshooting-wise. You're searching for how there can be something wrong with safety ground. (there's NEVER anything wrong with properly connected safety ground, but being a novice, you don't know that).

But it's an appealing fiction because it lets you bring the device online without hassles of procuring a non-defective one

Hooking up ground is always correct and it should be done FIRST. What that does is force you onto the correct diagnostic trail, instead of making you wheel-spin trying to figure out how ground could be defective.

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    I will just say the order of connection makes no difference as they should all completed in a deenergized state then power applied. – Ed Beal Jan 1 at 1:45
  • @EdBeal Sure, but then, when it doesn't work, someone starts unhooking wires. My message is in that case, ground is always connected. – Harper - Reinstate Monica Jan 1 at 3:37

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