Testing a FailSafe with a Fluke MM

[Solstice]

I have a 30A ProMariner FS that I test every season. I just tested mine and got an odd result....when testing diodes in one direction I got a good reading and in line with what I’ve historically seen. However when I tested the reverse (reversing contacts), I got a good reading, however my MM beeped the whole time (you’re supposed to keep the contacts on for a minute or so).
What is different is the MM—Since my last test I’m now using a Fluke 115. I have used it many times since Spring so I know it’s working.
So my question is anyone with a Fluke or similar weigh in on whether this is normal. Just to emphasize, the readings are good and no ‘OL’ ....thanks....

 

[Mark Maulden]

What are your good readings?

 

[Solstice]

.8 one way and .74 on the reverse (the side where I get the beeping)...

 

[Mark Maulden]

.8/.74 what? Are you testing DC voltage drop when powered or resistance unpowered? I think this device might be an isolator of some kind? So I doubt you are measuring DC voltage drop. So I’m assuming you are measuring resistance across a single diode or a series of diodes. What is your meter set to? A simple schematic and test points would help

 

[Brian D]

If you are measuring DC resistance across a diode, in one direction you will get some value. In the reverse direction you will get "OL". If you get 0 (zero) there is an issue with the diode. Diodes can also be used for voltage drop.

 

[Solstice]

The meter is set to Diode test and it is showing DC voltage through the diodes. Anything below 1 and not open (OL) shows a working state.
This is a common test for Galvanic Isolators. My issue is that with the Fluke I get a beeping when testing the diodes in the opposite side. You test with connectors on each side of the GI then switch the leads.
With my last MM I would get essentially the same results I got today without the beeping...

 

[Mark Maulden]

Page 15 of the manual. Sorry could not paste it here. You have a bad diode assuming no other circuitry is involved. Or even connected

 

[Solstice]

Page 15 of the manual. Sorry could not paste it here. You have a bad diode assuming no other circuitry is involved. Or even connected

Thanks Mark,
I have the manual and although it says diodes are bad if you get an open load, the reason I am confused is because I’m getting a reading less than 5% of the other way. The beeping is why I suspect that you may be right.
I will use another MM to verify. I just wanted to make sure this wasn’t something unique to the Fluke which is the only variable that’s changed over the last 8 years.
Everything gets disconnected when I do the test...thanks

 

[Mark Maulden]

Instead of another meter, just the resistance function. Open one way and some number the other way

 

[Ken Cross]

When using any digital multimeter you have a diode scale because the voltage in a resistance setting is low enough that it will not bias a diode so readings are simply open or nearly so.

Using the diode scale it produces a constant current and the voltage shown is what is required to produce that constant current. This is much different than a resistance measurement. The sound is simply a beeper for checking continuity and doesn't mean much in terms of bias voltage.

Typically a silicone diode forward bias will be around 0.6 to 0.7 volts. Reverse bias will appear open circuit.

Ken

 

[Solstice]

Just want to follow up and provide the solution for anyone who may come across this in the future...
As I mentioned I regularly test my GI and had an odd result when testing in Reverse this time. Although I don’t know why it was different this year (other than a new MM), but it turns out you must De-energize the capacitors before every test.
The capacitors will retain a charge until they are shorted by simply taking a wire and contacting both studs or briefly using the MM in DC voltage mode. Of course everything should be disconnected from AC before testing. Turns out my GI is just fine!
Hope this helps someone going forward....

 

[Brian D]

Discharging capacitors can be a stressful event. It doesn't have to be. You can use a light bulb with alligator clips so there is no sudden inrush of current. A 100Ω resistor rated at 5watts can also be used.

Anyway, thanks for the update. I really don't like it when someone post they solved the issue and never say what they did. Or even not say anything at all.

Bravo Zulu