Transformer Grounding


Thread Starter


I have an ACME control transformer. I have connected the primary with two phases, and the output I took 230V from secondary. When I measure across the secondary terminals, I get 230V. When I measure one of the secondary with the earth terminal, I get 150v or may be less and measuring the other terminal with the earth terminal of the transformer I see more than 70v. This was confusing me so I measured from my control panel across the neutral and earth terminal and found to have more than 80v.

I have a question, if i connect one of the secondary terminal to the earth terminal of the transformer will i get 230V and solve my issue?
There are several different ways of wiring a control transformer. Normally in North America they are 120 Volts with one side grounded but your 230 Volts, I'm not sure of, I can't imagine its done for safety reasons. Are both sides of the transformer secondary fused? if not, I would expect the un-fused side to be grounded.

Sometimes you see an ungrounded secondary with 2 lamps in series with the center point grounded to show when there is a ground fault, but this scheme can be difficult to troubleshoot.

What sort of loads is the transformer feeding?
Does it have a single phase or 2 pole distribution board?

Since you seen to be the designer make sure it lines up with your relevant electrical code.

I"ll explain the scenario. It is associated with a batching plant control panel. From the main power panel towards the batching control panel, two phases( 400V ) are connected to an ACME transformer, and the output taken as 230V from the transformer to power the control panel and the relay modules and other components. The transformer has earth point and is grounded. The output 230V from the transformer when measured across the output terminals shows 230V, but when measured with a terminal and earth it shows 150V or less and with other terminal to earth of transformer shows 70 to 110V. As I said, this 230V is powering the control panel and devices. Also this 230V is feeding to an isolation transformer which is feeding a sub panel of the batching system where the loadcell modules and other electronic cards are connected. when measured across the isolation transformer output terminals the same situation seen as above.

I had an issue with loadcells several times. The loadcells lock down without any values. So i checked the earthing and thus came to see this problem. I’m doubting whether due to this reason with the transformer side im facing the issue with load cells.

One terminal of the secondary of your transformer must be earthed and then your problem will get solve.
Even though you are using 2 phases of 400 Volts. It's still single phase to the transformer. If the secondary is grounded at a 150V tap then the other side must be 230 - 150 = 80 Volts not 70 - 110 unless you have something loose. Find out why you are getting an erratic Voltage.

I think you are saying each end of the 230V secondary is going to fuses or breakers feeding 115V loads or perhaps a 2 pole breaker feeding a 230V load (similar to Nth American house wiring). Check the markings on the transformer. I'm wondering if the Voltages you see are caused by unequal loads to Neutral and the tap to load neutrals is open.

Different Scenario
If you lose the power supply feeding the load cells their output will go to zero even if they are fully loaded. There are really too many variations in weighing systems for me to guess what you have, but I can say the output of the cells should never go to zero if there is weight on them.

Is there any way you can post a schematic showing the transformer secondary wiring?
Hello Roy,

I have the winding diagram of the transformer. How can i post here? I don't see any option to post an image.

<b>Moderator's note:</b> at present, you cannot post images on the site. You can post the images to a share like google docs and post the URL here.

If one lead in the secondary is not grounded, and you are getting some voltage with one lead to ground, It is only because of capacitance & leakage current to the body. Just ground one lead in secondary.
Not so fast, lets see the schematic

I use a free hosting service, seems to work well at the moment.
BTW, I think this may be the cause of the other thread
"Sudden Lockdown of Loadcell Values" (

>If one lead in the secondary is not grounded, and you are
>getting some voltage with one lead to ground, It is only
>because of capacitance & leakage current to the body. Just
>ground one lead in secondary.

I also had same issue with my control panel.

request you to ensure proper earthing & try out with the measurement with analog meters. Then you'll get the correct reading. At present voltage observed between the neutral & earth may be false voltage, just try out with analog meters.

The drawing you posted is only the transformer. It doesn't show how it's wired, that's up to whoever designed the system. Please post which secondary terminals are jumpered. Where is the ground connected?

>Please go through the image

Do you have all single pole 230 Volt loads or are some of the loads 115 Volt? If the former, I would expect to see the unfused end grounded. If the latter, I would expect to see fuses at both X4 and X1 with X3/X2 connected to the neutral terminals and ground.

Can you post a drawing or sketch showing the secondary Voltage distribution.
Please advise where in the world you are.
According to your transformer nameplate, you have a 240 VAC transformer, assuming on the primary side that you connected H3 to H6 and use H1 & H8 as primary input lines (400 VAC), and on the secondary side, you connected X2 to X3 and use X1 and X4 as secondary output lines (240 VAC). If you need a 120 VAC or a 120/240 secondary, you will have hook the secondary up differently.

You may have a ground connection in your transformer but that does not mean the secondary side is grounded (e.g. not connected to transformer secondary and/or connection not connected to plant ground grid). The fact that you got an odd reading on the voltage to ground probably indicates that the transformer is probably not grounded and you are reading a capacitive coupling voltage to ground probably due to the high input impedance of a DVM (this is one way you can typically tell if a circuit is not grounded). You may not be able to get the same reading with an analog meter as indicated by one of the other posters because of the meter impedance.

You need to follow the electrical code in your country in regards to grounding the transformer. Normally I would expect one side of the transformer to be grounded (X4) and you would have 240 VAC to X4/ground (though for 240 VAC to ground probably not required to be per US NEC 250.20/21).

William (Bill) L. Mostia, Jr. PE
ISA Fellow, FS Eng. (TUV Rheinland)
WLM Engineering Co.

I tried the method you suggested but that didn't solve my problem. The neutral to ground voltage was around 80v. I have two transformers in this scenario and I have grounded the secondaries of these transformers but still when i check the neutral to ground terminals it shows this voltage.

Now about the earthing, the earth pit was constructed new due to this issue and the earthing was done separately. Still the issue persist.

>BTW, I think this may be the cause of the other thread
>"Sudden Lockdown of Loadcell Values"
>>If one lead in the secondary is not grounded, and you are
>>getting some voltage with one lead to ground, It is only
>>because of capacitance & leakage current to the body. Just
>>ground one lead in secondary.
Assuming that you refer to power transformers, some manufacturing facilities use what is call a floating ground with the neutral taken to ground through a resistor with suitable resistance and power rating.

The purpose of this resistor is to identify currents associated with load imbalances or insulation degeneration, where you have a large number of motors involved in an assembly line sort of system.