Mains protection

[vk6zgo]

Be aware that a DMM without low-Z mode is the wrong tool for mains because the meter's high input impedance and capacitive coupling between wires can screw up the measurement. The best tool for that is a Duspol or a similar tester.

I'm sure that between my 87V and my 289 I have one with it. But thank you anyway for the info.

no Low-Z mode for 87V, but 289 has it (the first far left switch setting) 

It's a fixed 0 to 1000 volt range only (no auto or manual range) with both AC and DC readouts, and gets it done   
 
Take note that in Low-Z mode with any meter, you will trip the RCD/GFCI if probing between a line and earth ground
which may score you nil readings and some nasty vibes from others on the premises,
how do I know this...?   

I found that out with an Avo Model 8.

On one particular piece of equipment, the Mains wiring was a bit overgrown with other components, so as a rough check for the presence of Mains, it was easier to just measure between Active & Earth.(chassis).

Unbeknownst to me, the bench circuit breakers had been upgraded & now included RCDs.
The breaker tripped, I reset it & re-measured------ the breaker tripped again.

I then checked the logbook, which I should have done before---sure enough, the breaker upgrade was logged!

[HalFET]

A large piece of polycarbonate is my main advice when you're going to be messing with power stages, I still have a scar from an IGBT that decided it didn't belong in this world. It could have been a lot worse if I didn't have a polycarbonate shield in front of my face. Also good to have around is a few PTFE stand offs to put the chassis on, that way if the chassis goes hot it at least won't be in contact with your bench (and ESD protection).

[Electro Detective]

It's not a bug, it's a feature called RCD test.

It's a 'FREE' RCD test with a 3k resistance, I forget what the actual mA trip current is at 240 volts 
but close enough for a quick and dirty go/no go test, as I don't trust nor assume anything with the RCD onboard TEST button,

Low-Z comes in handy when the proper bells and whistles RCD tester is in the wrong tool bag.. miles away 

I can also relate to a similar experience to vk6zgo with an AVO 8, using it to find out why the 'not so faulty' Fluke with Low-Z tripped the power 


Hey, you gotta do these things at least once to learn not to rush about and be overconfident..   

Though the experience is loads better, if no one is around to share it with you at the time..   

[Monkeh]

It's not a bug, it's a feature called RCD test.

It's a 'FREE' RCD test with a 3k resistance, I forget what the actual mA trip current is at 240 volts 
but close enough for a quick and dirty go/no go test, as I don't trust nor assume anything with the RCD onboard TEST button,

Low-Z comes in handy when the proper bells and whistles RCD tester is in the wrong tool bag.. miles away 

I can also relate to a similar experience to vk6zgo with an AVO 8, using it to find out why the 'not so faulty' Fluke with Low-Z tripped the power 


Hey, you gotta do these things at least once to learn not to rush about and be overconfident..   

Though the experience is loads better, if no one is around to share it with you at the time..   

The test button does exactly the same thing. Neither the test button nor your meter is a reliable test of an RCD.

[Electro Detective]

The test button does exactly the same thing. Neither the test button nor your meter is a reliable test of an RCD.

As stated previously, it's just a convenient go/no go test to see if the unit actually trips from various locations,
rather than having faith that just hitting on the TEST button is enough, and assume the premises have perfect wiring and no dramas at all the outlets

FWIW I've replaced 4 pole RCDs, used in 3 phase and single phase apps, that tripped fine from the outlets, yet the onboard TEST button failed to trip the unit,
and vice versa with other units. And no, it wasn't a Neutral mis-wire, which I always check.   

Merely replacing those units with the same type/model, and wired in exactly as they were, fixed all issues
and passed any/all testing procedures with correct specified mA/Time/Polarity specs 

i.e. a Low-Z meter, AVO 8 or 7  or Duspol rig or tacky taped up DIY globe poker thingie   
can get you started good enough on a job to diagnose a faulty or suss RCD/GFCI

Yes it's best    to have a proper working RCD tester (or two) to test and check actual performance,
but if it's not there in a surprise situation, the other alternatives can get you started,
or at the least to verify a new replacement RCD/GFCI works
 

[Messtechniker]

Everyday Practical Electronics (now just Practical Electronics) had a project for a differential probe a short while ago. I built it and it works very well. Quite cheap too (a few tens of pounds I recall).

If anyone interested and can't find it, just shout and I'll dig out the reference.

Just on the last leg of putting two together. Since the TLE2022 is not available any more I now use an AD 822 AN as the output opamp. Also tried a TL 072, NE 5532. However, the AD 822 gave the best match with respect the published frequency response curves. .

Btw: it was great to have a lab power supply with 3 separate outputs (HMC 8043). Two 9 V outputs for the isolating high voltage probe and one 12 V output for the XR 2206 frequency generator. Had to use it since I am still waiting for the UNI-T UTG962 to become available on the market.