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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.

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    \$\begingroup\$ My DMM will take Insulated Banana Plugs If you use these and connect them to your load with suitable insulation you should be OK. \$\endgroup\$
    – Warren Hill
    Commented Jul 24, 2018 at 16:37
  • \$\begingroup\$ The jacks I am adding in are 2mm jacks meant to be inserted with the leads of a DMM. So there is not really a standard banana jack that you can install. \$\endgroup\$
    – DIODEX
    Commented Jul 24, 2018 at 16:40
  • \$\begingroup\$ What does "outside" mean in this context: "The only components that can be touched from the outside are two 2mm banana jacks" \$\endgroup\$
    – Andy aka
    Commented Jul 24, 2018 at 17:12
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    \$\begingroup\$ Have a window in your fixture with a digital panel meter inside and viewable. They are not very expensive and anyway, what price for safety? \$\endgroup\$
    – Andy aka
    Commented Jul 24, 2018 at 17:46
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    \$\begingroup\$ @DIODEX - "one person could potentially grab the lead by the metal piece and insert it into the jack and get shocked". Oh. So you're trying to make it foolproof. That's not going to work in the long run, but you can try. Put two momentary switches (normally open) in series with the jack. Place the two switches far enough apart that they cannot be operated with one hand. Then the operator will need to insert the probe and let go in order to operate both switches. "It is difficult to make things foolproof, since fools can be so very clever." \$\endgroup\$
    – WhatRoughBeast
    Commented Jul 24, 2018 at 19:40

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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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  • \$\begingroup\$ I was planning to use 2x 90K resistors and 1x 20K resistor. At 200V the current allowed would be 1mA. The voltage across the middle 20K resistor (in the middle of the 90K resistors), would be 20V. \$\endgroup\$
    – DIODEX
    Commented Jul 24, 2018 at 16:49
  • \$\begingroup\$ if you do this, consider placing a 28V zener across the 20k. Now even if one of the 90k resistors fails short (extremely unlikely, unless the physical design is pretty bad, resistors virtually always fail open) or the 20k goes open (more likely) you can only have 28V DC at the terminals. \$\endgroup\$
    – danmcb
    Commented Jul 24, 2018 at 19:56

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