Electronic – Measuring 200 volts safely

acmultimetervoltagevoltage dividervoltage measurement

While I can measure line voltage safely, I need to write a procedure to have someone else measure the voltage. The load is pure resistive,and I want to measure the voltage across it, with a battery powered DMM.

While my main idea was to place a transformer to isolate the measurement and lower the voltage being measured, I did not want to add the inductance of the transformer to the load..

The only components that can be touched from the outside are two 2mm banana jacks. My question is, would it be safe to just to drop the voltage into the banana jacks via 3 high ohm resistor divider to measure 200Vac (line to line).

The main issue I see is if one of the resistors somehow shorts out, and a person grabs the DMM lead by the metal inserts into the jack.

Best Answer

I see two major factors to consider when implementing a resistive divider for this purpose.

  1. You are likely to need fairly large resistors to keep the potential "touch currents" down to safe levels. Remember to consider not just normal operation but the case where one side of the line is down and the other is not. On the other side rember to take acount of the fact your multimeter has finite input resistance (In my experiance nearly all are 10 megohm). Hopefully you can find resistor values that are high enough to protect the user and yet low enough to allow good readings.
  2. Resistors can fail, therefore to provide "single fault safety" you should be using at least two resistors in series to implement each logicial resistor each of which is rated to take the full line voltage in the event of a fault.

If you have the space for it I would suggest putting some kind of shrouded connector on the panel. Then providing a suitable lead or leads to connect it to the multimeter. It might be an idea to do this in addition to the resistor set-up to provide two layers of protection.

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