Oil burner transformers are step-up transformers. A step-up transformer takes one voltage, the primary winding voltage, and raises it to a higher, secondary voltage. The oil burner transformer raises the 120 Volt line voltage to 10,000 Volts or even higher to ignite the fuel oil/air mixture, much as spark plugs ignite the gas/air mixture in your car engine's cylinders.
Working with this kind of voltage is dangerous. Unlike the 120 Volts that you have encountered when working with lights and receptacle outlets, you don't have to actually touch a live part to be shocked. High voltage can actually leap across a sizeable air gap and shock you. Proceed with caution.
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Things you need
- DMM (digital multimeter)
- HV (high voltage) test probe
- 110 Volt test jumper cord
- Electrician's screwdrivers
- Large, well insulated screwdriver
- Electrician's nut-drivers
- 6 inch adjustable wrench
- Flashlight or work light
Turn the oil burner circuit breaker at the service panel and the oil burner safety-disconnect switch at the oil burner's location. The oil burner's safety-disconnect switch may look like a light switch but will have a red cover. It should have "oil burner" printed on it too.
Open the oil burner's service door or remove the service panel. Locate the oil burner transformer and remove the cover to the electrical connection box attached to it.
Set your DMM to the AC Volts range and check for voltage at the transformer's primary terminals. Depending on the transformer's design, the primary terminals may be screws or they may be a set of black pigtail leads connected to the line wires with wire nuts. If the latter, carefully remove the wire nuts without touching the bare conductors and then touch the meter probes to the line wires. With both the safety-switch and the circuit breaker turned off, the meter should read "0" volts.
Reconnect the primary leads.
Remove the transformer from its support, moving it so you can see the secondary winding terminals. These HV (high voltage) terminals will be on porcelain standoffs.
Inspect the two porcelain standoffs, and insulators, for cracks or other damage. It they are cracked or chipped, you must replace them before continuing with the transformer test. Moisture build up in the cracks will allow the HV to short out to ground.
Disconnect the power leads to the pump motor to keep the pump from spraying fuel oil during the test. Failure to follow this step can cause a fire.
Turn on the oil burner's branch circuit breaker and turn on the oil burner's safety-disconnect switch.
Set the room thermostat to its normal setting.
Verify the presence of line voltage with your DMM. You have about 15 seconds to make this test because after 15 seconds has elapsed, the oil burner's safety lockout will engage disconnecting all power to the system. If you have to, turn off the safety-disconnect switch, wait one minute for the lockout to reset, and then test again.
Attach the HV probe ground clip to the transformer's metal case and touch the tip of the probe to the HV terminals. The meter should indicate between 5,000 Volts from each terminal to ground. Again, you have roughly 15 seconds to complete this test before the safety lockout engages.
A rough test of the condition of an oil burner transformer can be done with a heavy, well-insulated screwdriver. With the transformer energised, place the screwdriver's shaft against the energised HV terminal and slide the blade towards the metal case. A good transformer will produce a spark around 3/4-inches long.
If the HV reads below 5,000 Volts to ground or the spark seems weak, replace the transformer.
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