Microwave oven repair safety precautions

Electrocution by microwave oven

Microwave oven repair safety precautions

What is more dangerous, the overhead line of a train or the inside of your microwave oven? It is the microwave oven, because of the higher voltage that is used. Take the danger serious; in the US, 4 people per year die from electrocution trying to repair microwave ovens, even electricians by profession. Here are some victims:

1989 James D. Batson 40 year, Porter County USA
2000 Mike Greenwell 33 year, Kenosha USA
2008 Charles N. Martin 28 year, King William USA, father of four children
2009 Juan Carlos Flores 31 year, Edinburg UK
2012 Renaldo Odelio-Enriquez Samayoa 32 year, Roanoke Rapids USA
2012 Jimmy Roddy 25 year, Sylvester Georgia USA, caused by the capacitor voltage, the microwave oven was not plugged in
2012 Philip Dodd 62-year, Royal Tunbridge Wells UK, electrician at Marks & Spencer
2014 Leslaw Pawlica, 33 year, Chicago USA

Electrocuted by microwave oven high voltage transformer

Five times as dangerous as the overhead line of a train!

Microwave oven is 5 times as dangerous as the overhead line of a train!

The high voltage inside a microwave oven is 3400Vrms. Because the power is proportional to the square of the voltage, it is 5 times as dangerous as the 1500V overhead line of a train. Note that an electric chair uses also just 1500V. In contrast to HV circuits inside low power electronic products, which are not lethal, you will normally NOT survive the high voltage of a microwave oven, the available power of more than 1000W is enough to kill you instantly. 

Safety guideline for repair of microwave ovens

• Work with extra attention, not if you are tired.
• Note that normal insulated tools are not safe, the high voltage may strike through it. 
• Don't do any measurements at the high voltage, this is deadly; the high voltage passes through the insulation of the probe easily. (However, we still use them here for extra safety).
• Unplug the microwave oven first. However, a microwave oven must always be earthed, so connect the metal chassis with a separate cable with the earth pin in the wall socket
• Wear electrical insulating rubber gloves, although this does not protect against the 4000V.
• Use installation pliers to perform the following tasks:
• Discharge the high voltage capacitor, as described below, because the charge may be present long time after the microwave oven is unplugged; some people have been died by this charge. Don't trust on the internal bleeder resistor, it can fail.
• Connect two decent alligator clip test leads to the microwave metal chassis ground. Be sure that the leads aren't broken, measure this first!
• Clip a resistor of 1K ... 1M to the other side of a test lead and discharge the two terminals of the capacitor one by one via the resistor. Use the installation pliers, which is grounded. The resistor is not critical because it is just to prevent sparks. Note that a resistor of 1MΩ discharges the capacitor of 1μF in 3 seconds.

Discharging the microwave oven capacitor

• Connect each grounded clip at the terminal of the capacitor directly (use the installation pliers!).

Grounding the microwave oven capacitor

• Remain the clip leads connected all the time during the microwave oven repair.
• Disconnect one of the high voltage transformer leads on the mains side.

Disconnect mains side

• Now you can further repair and troubleshoot without danger because the high voltage transformer is disconnected.

The waveform of the microwave oven voltage

The high voltage at the microwave tube is the sum of a sine wave of 2000Vrms and a 2800V DC voltage, see the waveform:

Microwave oven high voltage waveform

The unloaded RMS voltage is √(2800V^2 + (2800V^2) / 2) = 3400V.