Control Transformer Grounding


Thread Starter


I have a control transformer with 3 phase 480V primary and 24VAC secondary. When I measure between ground and any of the two points of the 24VAC secondary side I get around 110 volts. Right now I don't have the secondary grounded. Could that be it?
I am not familiar with control transformers, but I believe that you need to measure voltage between the phases of the 24VAC but not between it and ground. It is easiest to use a battery operated voltmeter that can measure AC voltage. If you are using an oscilloscope it is trickier as the scope probe's ground clip is connected to the AC safety ground [or Earth ground] via the scope internal power supply & chassis. Refer to your scope manual for AC voltage measurement. Be careful not to exceed the voltage ratings of your equipment. The thing to remember is that you need a differential set of probes to do high voltage AC measurements or use two channels with the scope set for differential mode or math ch1-ch2 [if the scope has this capability]. Do not exceed the AC voltage ratings of your scope channels and probes. As for grounding the transformer secondary I would refer to the manufacturer's application instructions.

Be careful and good luck.

Oly Jeon-Chapman

Curt Wuollet

Yes, especially if you are using a high impedance meter. There is always some capacitive coupling and leakage.



Free Tools!
Machine Automation Tools (LinuxPLC) Free, Truly Open & Publicly Owned
Industrial Automation Software For Linux.
Day Job: Heartland Engineering, Automation & ATE for Automotive
Consultancy: Wide Open Technologies: Moving Business & Automation to

YES! You can get digests, or limit messages by topic. See for details.
Before posting, please read
The Automation List is managed by Inc.


Hunter Farris

It has always been a common practice in the industry that I work in to ground the neutral on the secondary of the CPT to prevend voltage fluctuation as well as the problem that you describe. This will also help with safety of system.

Hunter Farris

Carlos Malpartida

That is normal because, do not forget, you do not have the same voltage referencia, and the secondary only warranty the V between the
terminal, that is not good because some person will have a electric shock besides the potential diference, i recommend to ground it but to
the control ground