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, firstname.lastname@example.org
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, Mwhite2690@aol.com
Response No. 4
I always grounded the X2 connection. This stabilized the control voltage for these
Joseph Wages Jr., email@example.com
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, firstname.lastname@example.org
Mike Holtís 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 deviceís rating.
This is accomplished by bonding the metal parts to the systemís 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 itís 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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