This article was posted before 01/01/2011 and is most likely outdated.

Grounding - Machine Tool Transformers (9-15-99)

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How and where should a dry type machine tool transformer with H1, H2, X1, and X2 terminals be grounded? Or, should it be left ungrounded? The transformers typically supply power to the components within the control panel but they also supply power to field-mounted loads and devices. These transformers are often enclosed in housing (some are of the open type) and the secondary voltage is generally 120V, but they can be 12V, 24V, 240V, and 480V.


Response No. 1

The transformer is automatically grounded when you screw the transformer housing to the control enclosure, which should be grounded. See the following Code Sections (1999 NEC) 250-112(d), 250-172, 250-178, 250-96(a)(b), 250-30, 430-96, 430-144, 450-10.


Also see NFPA 79, it has good information for your application and check with your manufacturer concerning this to verify. I hope this helps. I have a book on control transformers that shows this application.

David R. Carpenter,


Member Comment

Response No. 1 is wrong. You must ground one side of the secondary otherwise it will not recognize the ground because a transformer is a floating coil and it needs to be bonded to ground to complete the circuit or you will get weird volt readings as indicated in Response No. 2.

Response No. 2

I have a lot of these transformers in my fish processing plant. We provide a jumper from X2 to the main ground on the control panel. If you don't ground the control transformer you can't trouble shoot the component parts because you'll get a bunch of weird voltage readings.


Response No. 3

Machine tool wiring is not required to comply with the NEC because the components if the wiring is designed by an engineer who accepts the liability for the system. Not all machine tool transformers are or should be grounded. You need to check with the wiring schematics for the machine you are installing it in. If it shows the secondary side grounded, it will generally be grounded at X2. However some control systems have been designed to be ungrounded to protect them from stray currents, or other types of interference, and grounding them would not only be a bad idea, it would violate the listing of the machine.


If you can't tell by looking at the schematics whether grounding is appropriate or not I would suggest you call the manufacturer and talk to their service department. Usually this kind of question is very welcome as it can save a lot of trouble later on when the wrong procedure is followed. You should never change the designed wiring because to do so sets you up to be liable for the system.

Michael White,


Response No. 4

I always grounded the X2 connection. This stabilized the control voltage for these installations.

Joseph Wages Jr.,


Response No. 5

Generally since there is 120v potential between X1 and X2 on the secondary, X2 is used as the secondary grounded conductor, and equipment ground. Let me know if I am wrong. This is the way we wire our alarm panels, and secondary contact voltage on motor starters.

Ken Teverbaugh,


Mike Holt's Response

The purpose of grounding is to remove dangerous voltage from metal parts of the electrical system and/or building structure that occurs during a ground-fault. Failure to remove the dangerous voltage can create a condition where persons can be killed by electric shock and/or fire, not to mention power quality issues.


To remove dangerous voltage from metal parts, the circuit overcurrent protection device must open to clear the ground-fault. The effectiveness of the grounding path plays a vital role in the activation of the overcurrent protection device to clear the ground-fault. To open the protection device quickly, the impedance of the grounding path must be low enough to permit ground-fault current to reach a level of at least 3 times (preferably 5-10 times) the overcurrent protection device's rating.


This is accomplished by bonding the metal parts to the system's grounded conductor (X2), which provides the low impedance path necessary to facilitate the clearing of phase-to-ground faults by opening the circuit overcurrent protection device.


However NEC Section 250-20 states that alternating-current circuits of less than 50 volts must be grounded under any of the following conditions:

1. If the transformer supply system exceeds 150 volts to ground

2. If the transformer supply system is ungrounded

3. Where installed as overhead conductors outside of buildings


So the bottom line is unless it's designed by an electrical engineer, ground all systems that operate at over 50 volts by bonding the X2 terminal to the grounded metal enclosure.


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