I am planning on building a simple AC power monitor as shown in the attached figure. The 500 kΩ resistors are to limit the current out to the test terminals in case a hapless human (me) accidentally touched them (the 0.1 ohm shunt will be wired on the low side, but of course I could unknowingly plug it into an outlet with hot and neutral reversed). I chose 250 μA as a current limit because it is half the typical recommended maximum leakage current for AC operation and is well below the human current threshold. I could protect against bad polarity by wiring up three neon bulbs with current limiting resistors in the usual AC outlet tester way. If I can find a 25 W resistor, this should let me test to about 15 A.

To be a more safe device, the monitor lines could come through some transformers. But I don't feel this is necessary. I'd like to get the collective wisdom of the folks here about whether the pictured design is considered a reasonably safe design. It's going to go into a metal junction box and the jacks will be flush banana jacks. The input side might be connected to a DPST toggle switch.

Here's the reason why I think it's safe. The resistors would have to fail in a short to expose the user to a potentially hazardous voltage (and you'd have to stick a screwdriver down into the flush banana jacks). And, for a properly wired unit plugged into an outlet with the proper polarity, there's only one resistor short that would be hazardous -- the one connected to the hot on the voltage monitor. If it was plugged into an outlet with hot and neutral reversed, then three lines are potentially hazardous one of those three resistor shorts.

I'll use 1/4 W carbon film resistors that I have on hand.

Under what conditions can a resistor fail as a short? I've never heard of that happening, as long as the resistor was good to begin with. To be even more conservative, each resistor could be replaced by two 500 kΩ resistors in series; this greatly increases the reliability because two resistors in series both have to fail as a short.

This thing will get used infrequently and mostly indoors. My usual connections will be to the two channels of a scope so I can look at the current, voltage, and power waveforms.

 

It would be better to use a current transformer to measure the current as to isolate the measuring instrument from the mains voltage. Note that if a current transformer is not used carefully it can be dangerous.

 

The device will only measure true power (Watts) of purely resistive loads.

 

Allegro makes some nice current sensors, from 5A to 150A, they are isolated so you can work with low voltage.

On the voltage side you could also use a double winded isolation transformer. Touching either wire would be safe. Touching both would be dangerous. So what you do is have a terminal in this room, and have the other terminal in the next room. That way it would be very difficult for any one to touch both wires at the same time


A step down transformer could have you monitor 0 to 12V as to 0 to 120V. The 10 : 1 relationship will stay the same.

 

Thanks, folks. I know about and have isolation transformers, current clamps, current transformers, etc.

I'm still interested in knowing your opinion about whether my original proposal is too unsafe. Thus:

1. Do you see a hazard in the original design proposed?

2. Have any of you ever seen a carbon film resistor fail as a short (If possible, I'd like first-hand experience, not hearsay)? In other words, the resistor goes from some nominal resistance (here, half a meg) to one to a few tens of ohms.

The shorted resistor or an accidental short from a mains line touching the banana jack are the only potential failure modes I can see that would change this thing from safe to unsafe. The latter can be dealt with by strong mechanical design and good layout.

 

Dad ,

It's not about being unsafe or hazardous, but rather uncomfortable. There is a risk in any component failing, and things can go wrong.

If I was you I would omit those resistors, and install an earth leakage protection switch instead. You can switch it off when you work with the wiring and switch on when you're done.

That way you will form a habit of working correctly with power.

If there was another proper way it would have been in use.

 

Resistors have a maximum voltage rating. Check what it is for the resistors you will use and allow for a good margin then they will last and not short out due to excessive voltage.

You have to decide for yourself on the safety issue.

 

With no intrinsic safety, bad things can happen. If you can't use an isolation transformer (strongly advised), use a PB switch to hold the meter jacks disconnected until it is pressed for the test reading.