Step Down Transformer

I am not sure if I am posting this in the correct place but I am going to ask anyway.

Why does a step down transformer need to be connected to ground?

I had a problem where I was not able to read 120VAC from L1 to ground straight off the transformer.

The machine I was working on would energize solenoids but when I tried to energize contactor it would turn everthing off.

The only thing I did to correct it was install a wire from L2 to ground.

The only reason I connected it was because the green ground wire was laying at the bottom of the cabinet. I matched it up to the holes landed it and everthing worked??????????

Paul60033 said:

I am not sure if I am posting this in the correct place but I am going to ask anyway.

Why does a step down transformer need to be connected to ground?

I had a problem where I was not able to read 120VAC from L1 to ground straight off the transformer.

The machine I was working on would energize solenoids but when I tried to energize contactor it would turn everthing off.

The only thing I did to correct it was install a wire from L2 to ground.

The only reason I connected it was because the green ground wire was laying at the bottom of the cabinet. I matched it up to the holes landed it and everthing worked??????????

Click to expand...

You ground it to tie the transformer's low side to ground, and eliminate floating potentials.

Sounds like you should double check the contactor that wouldn't work, and make sure the L2 goes back to L2, and not directly to ground.

Technically, the neutral side of a control transformer does not HAVE to be grounded, but it is generally important to do so, for safety.

Also, no control circuit should be designed (anymore) that relies on a return to a system ground. All devices should return directly to the control source at the neutral point (L2 on a single phase, control transformer). In your case, it seems that there is probably more going on, as just 'connecting the green wire to where it seems to go' shouldn't make things happen.

Most likely, somewhere, you have a loose, or broken connection on the neutral side of the control system, which is common, as they are often 'daisy chained'.

I'd refer back to the system schematics, and try to find the root cause of the problem.

Paul60033 said:

The only reason I connected it was because the green ground wire was laying at the bottom of the cabinet. I matched it up to the holes landed it and everthing worked??????????

Click to expand...

I think we might be digging a little deep here. Wire fall out?

I went back to make sure L2 is contected to the contactor and it was not.
Someone has wire another L2 from a different transformer to this contactor.

This panel was built in 1981 and the schematics are no longer available. Poor maintenace record forces me to trace everthing with meter.

Is the other transformer grounded?

It is still a poor practice but would explain why the green wire found in the bottom of the cabinet would have caused it

Yes it is grounded

Technically if you are using one of the sides of a control transformer as a Grounded (neutral) conductor, it has to be grounded. If you are attaching a white (or natural gray) conductor to a control transformer and that point is not connected to the Grounding (earth) conductor, you would be in violation. Reason being is that white (or natural gray) is reserved for the Grounded conductor, and by definition the Grounded conductor is one that is intentionally grounded.

I think rdrast’s point is that the secondary of a control transformer (as generally used in our industry) does not have to be grounded. But as he pointed out, it is generally important to do so for safety.

Using a white conductor on an ungrounded secondary of a control transformer is a big .

A transformer has some leakage primary to secondary. The leakage is both capacitance and resistance. If your primary has a high potential above ground (most do), then the 120 secondary is also at a high voltage above ground (if it's ungrounded). Very small current, but it's there. Current goes higher as the transformer ages and the insulation breaks down. In case of a primary to secondary short, your 120 circuits would now be floating at the same potential the primary line is.
For these reasons and more, one side of the secondary goes to ground. Use a green wire for this. The current carrying conductors should be white. There should never be any current on a green wire, or the frame.

One example is an isolated copper buss bar. It has one lead from the transformer secondary, and the green ground wire. All other wires are white to the circuits.

Use an Amp Clamp on green wires. If any have current, fix it.

rdrast said:

I'd refer back to the system schematics, and try to find the root cause of the problem.

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I gotta hit on this one. This works great in a perfect society. My first plc project was to upgrade a robot welder from 16' of relay cabiinet to a PLC-2 with 3 relays and a motor starter. They did have the schematic. For 25 years it sat in a drawer. they added two motors, three addition welding heads, two conveyors, and a lot of other stuph. Still had the original print, but NO changes.

Years later I went into a plant, and in the cabinet of a smaller machine was a napkin. One side had the pplc program, and the other had the schematic. Nice, somewhat neat, convenient, and in color (not counting the ketchup/catsup stain).

Anyway, wierd ground paths hae caused a lot of problems over the years. Some machines will start running, others will quit, and then there is the shock potential of a hot metal frame that could have been safe if the secondary was grounded. I always ground the secondary and fuse the hot leg. I have seen countless systems with no fuses or CB's anywhere.
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Paul60033 said:

........the schematics are no longer available.

Poor maintenace record forces me to trace everthing with meter.

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Don't ya just hate when that happens?

I have chased wires by hand too many times.

Leadfoot said:

Don't ya just hate when that happens?

I have chased wires by hand too many times.

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Yep, I know the feeling. Still, just randomly connecting a wire because it 'Looks like it should go somewhere' is not a particularly valid (or safe) troubleshooting technique.

If there are no schematics on site, the you must indeed go and chase down the circuit by hand.

rdrast said:

Yep, I know the feeling. Still, just randomly connecting a wire because it 'Looks like it should go somewhere' is not a particularly valid (or safe) troubleshooting technique.

If there are no schematics on site, the you must indeed go and chase down the circuit by hand.

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I realize this is not safe practice but I also knew that there would be no harm. The Majority of transformers that I've worked with are grounded.

I just wanted to know why the transformer need to be grounded.

I have receievd alot of helpful information and I appreciate everone taking time to educate me.

Thank You

"If you don't ask you'll never know"

While the majority may be, there are also many instances of using a control transformer as an isolation transformer (for instance, supplying power to a fractional HP Non-isolated drive). And there are other instances where what appears to be a control transformer, is actually a buck/boost autotransformer. In either case, grounding a winding is definately not a good idea

Paul60033 said:

I realize this is not safe practice but I also knew that there would be no harm. The Majority of transformers that I've worked with are grounded.

I just wanted to know why the transformer need to be grounded.

I have receievd alot of helpful information and I appreciate everone taking time to educate me.

Thank You

"If you don't ask you'll never know"

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