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Title
Verifying Isolation Transformer Output Current / Fixture Input Current
URL Name
Verifying-Isolation-Transformer-Output-Current-Fixture-Input-Current
Summary
Provides the procedure to verify that power is reaching the fixture by measuring isolation transformer output current and fixture input current using a clamp meter.
Owner
Mike Sotelo

Question

How do I verify isolation transformer output current?

How do I check fixture input current on an airfield lighting circuit?

What tool is required to measure isolation transformer secondary current?

Why should I clamp only one conductor during current measurement?

What happens if I clamp both conductors with the meter?

Why avoid clamp tests on molded connectors?

When should I use a known good fixture as a reference?

What step should the CCR be set to for this verification?

What is the expected clamp meter reading during the test?

Where should I locate the fixture pigtail cable?

How do I choose the best location to clamp the conductor?

What safety precautions apply when using a clamp meter on airfield circuits?

Is PPE required for verifying fixture input current?

What does it indicate if the clamp meter does not match the CCR setting?

Why confirm power is reaching the fixture before further diagnostics?

Content

Purpose

Before continuing with diagnostics, confirm that power is reaching the fixture.
Use the procedure below to verify isolation transformer output/fixture input current.


Tools Required

Clamp meter (True RMS, rated for airfield current levels).


Procedure

Locate the fixture’s pigtail cable — it connects to the transformer’s 2-prong female secondary.
Identify the two conductors leading into the molded plug.
Clamp your AC meter around one conductor only, not both.
Choose the straightest section of wire near the plug for best accuracy.
Power on the Constant Current Regulator (CCR) and set it to a known step, such as 6.6 A.
Observe the clamp meter reading.


Important Information

Clamp only one conductor to prevent phase cancellation.
Never attempt clamp tests on molded connectors where conductors are inaccessible.


Expected Result

The clamp meter reading should match the CCR output current setting.

Internal Notes

Digital Engagement Response

Article Created Date
2/19/2026, 7:10 PM
Publication Status
Published
Created Date
3/24/2026, 4:26 PM
Article Total View Count
147
Created By
Mike Sotelo, 3/24/2026, 4:26 PM
Archived Date
Last Modified Date
3/24/2026, 4:28 PM
Last Modified By
Mike Sotelo, 3/24/2026, 4:28 PM
Last Published Date
3/24/2026, 4:28 PM

Assigned To
Assigned By
Assignment Due Date
Assignment Date
Assignment Note
Troubleshooting Test

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Verifying Isolation Transformer Output Current / Fixture Input Current