Insulation tester vs Varistor - Page 1
Forum section: electronics / beginners

author: crgarcia
author_rank: Regular Contributor
timestamp: May 15, 2022, 08:48:19 pm
content: I found a UT511 super cheap in an online store (~$10) and I could not hold myself, had to buy ithttps://cdn-reichelt.de/documents/datenblatt/D100/UT511_MANUAL_english.pdfIf my research is correct, this thing can produce very high voltages and test if there is any leakage.My line of thinking was that I could use it to test if I my PCB is safe, or if it produces arcs when there are high voltages (testing the creepage/clearance/etc)The problem is that my PCB has a varistor and a fuse.What is going to happen if I try to measure isolation in between the red and blue boxes?Is it going to blow the Varistor?How much current can this device produce?Did I understand correctly what this device can do? or I got it all wrong?Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2022, 09:02:40 pm
content: Why would you want to measure the insulation between the live and neutral? It's a meaningless measurement and would risk blowing up the device.If this is going to be installed in a building, where an electrician will perform on insulation test on the wiring, they should disconnect it first.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 15, 2022, 09:15:52 pm
content: Yup! Why would you want to do that. The board must conduct between L and N otherwise there is no power and the insulation tester would detect extremely high resistance.

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2022, 09:42:44 pm
content: An insulation tester is designed to do just that: test the strength of the insulation i.e. part of the device which shouldn't conduct electricity.A valid application in this circuit would be to connect the live and neutral together to one probe and the 0V on the 5V side to the other probe.

author: crgarcia
author_rank: Regular Contributor
timestamp: May 15, 2022, 10:50:53 pm
content: Thank you for your answers! I think I have a much better understanding now(still long way to go! )Just out of curiosity, If I would have connected, would the Varistor short? Or do I need anything else than V to make it short?Another part of my circuit uses optocouplers to sense mains.If I understand correctly, I could test insulation between red and blue parts?In that part of the PCB, I am interested in testing the clearance and crepage. To validate that contacts will not ARC under high voltagesThis is my full project:https://github.com/crgarcia12/electronics-homeassistant-lightscontroll/tree/main/PCB/schematics-v17-safety-jlcpcbThanks!

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 15, 2022, 10:53:28 pm
content: It's interesting. I think the varistor would be shorted but don't you have a spare varistor? If so just put the varistor accross the meter probes and find out.

author: srb1954
author_rank: Super Contributor
timestamp: May 16, 2022, 12:38:11 am
content: Quote from: crgarcia on May 15, 2022, 10:50:53 pmNo. The varistor would have merely clamped the insulation resistance tester output to its rated clamping voltage. The output current (approx 1mA) from an insulation resistance tester is too low to damage the varistor. The tester would have likely indicated a reduced leakage resistance in the range of several megohms.QuoteLots of current (>10A) and voltage (>1kV) will damage a varistor. A high power surge tester would provide those but you don't normally see those outside of professional product testing labsQuoteYes. Connect all the input leads together on one side and short across the opto-transistor collector emitter on the other side before applying the test voltage. This is to prevent any capacitively coupled surge currents going through the delicate opto devices and damaging their junctions.QuoteA standard insulation resistance tester applies too low a voltage to verify creepage distances are OK. You need a device called a hi-pot tester, which applies up to 5kV, to properly test creepage.

author: srb1954
author_rank: Super Contributor
timestamp: May 16, 2022, 12:52:34 am
content: Quote from: Zero999 on May 15, 2022, 09:02:40 pmYou might do this to test the integrity of an across-the-line interference suppression capacitor inside a mains inlet filter. In this case the mains switch would have to be turned off to isolate the rest of the circuit to avoid false readings.Any mains powered device that can't stand testing by an insulation resistance tester of the appropriate test voltage (500V on 230VAC circuits) wouldn't last too long when connected to the mains supply. There are voltage spikes on the mains supply that regularly exceed 500V peak plus there is a lot more current and energy available from the mains supply to do damage. The application of an insulation resistance tester is a mild stress compared to the average mains connection.

author: rsjsouza
author_rank: Super Contributor
timestamp: May 16, 2022, 02:43:50 am
content: I have an UT513, which is the same device but with higher voltage. As others have said, it does not make much sense to know the "insulation" when testing your PCB through its two power circuits, but instead testing the insulation between your board and any conductive surrounding, such as a metallic housing, the earth pin, etc.Quote from: crgarcia on May 15, 2022, 10:50:53 pmAs others have mentioned, the UT511 does not have enough power to damage a varistor and would probably self-shutoff due to the excessive current draw if you are using higher voltages (1kV). The manual contains a warning about the minimum resistance for each range in order to prevent damages to the equipment.(a varistor can be easily damaged with voltage and current above its specs, but it must be sustained and not a transient. It also depends on the varistor)Quote from: crgarcia on May 15, 2022, 10:50:53 pmYes, you can. If the Opto can't withstand the voltage, it will be pierced internally and should be considered as destroyed. Throw it in the trash can.One additional interesting use for the insulation testing is to check for leakage in high voltage capacitors - a great use given that usually we lack high voltage sources to test such important specification. I did a short video (in portuguese, sorry) about that:https://youtu.be/uvl3Yo2AjGg