Testing low resistance at high voltage - Page 1
Forum section: electronics / testgear

author: ion
author_rank: Regular Contributor
timestamp: March 15, 2018, 11:32:34 am
content: Lately I had a problem with a hotplate were it kept blowing fuses/tripping the circuit breaker when plugged in but testing the circuit with a multimeter showed no problems.I eventually narrowed it down to the heating elements, but the resistance was around what it should have been. I figured the insulation was compromised, but not enough to cause problems at the low voltage from the multimeter. I also hooked it up to the bench supply but again it worked fine cranked up to 30V.Not having a safe way to test at 240V I searched ebay for a cheap insulation tester and with the BM500A at under $20, I figured it would be worth a shot. Didn't need it in the end, as some less than gentle probing cause the insulation to fail completely, making the fault location obvious.However, checking the specs of the BM500A, I noticed it has a resolution of 1 Kohm. Some of the other (afforadable) testers are higher - 100K or even >1 Megaohm. So how do you use these to test for a short circuit, were the resistance under normal conditions is a few hundred ohms?

author: SG-1
author_rank: Regular Contributor
timestamp: March 15, 2018, 01:16:59 pm
content: Connect one of the meters probes to the metal frame of the hot plate & the other lead tobothends of the element. You are looking for a insulation failure between the insulation & frame, not within the element itself. The element should be completely insulated from the plate or frame.

author: ion
author_rank: Regular Contributor
timestamp: March 15, 2018, 01:47:02 pm
content: I understand that's how the insulation test is normally done, but in this case I think the short was between the live and neutral, just before the element.Besides, all the metalwork in the hotplate is earthed - wouldn't it have tripped the RCD rather than the fuse/breaker? I think the one test I didn't do was powering it with the earth disconnected, so I can't be 100% sure.Edit: Just to be clear, it's this style of heating element:https://www.ebay.com/sch/i.html?_odkw=120Wcartridge+heating+element&_nkw=120W+cartridge+heating+element&_sacat=0The failure point was were the wires enter the metal cartridge.

author: SG-1
author_rank: Regular Contributor
timestamp: March 16, 2018, 02:52:38 am
content: I do not see any hope using the insulation tester between hot & neutral. For that you need a variable ac supply so you can slowly increase the voltage until failure. it will have to deliver the operating current. I would put a high energy current limiting fuse in series to protect my ac supply from the short. The fuse will probably will cost more than the heater element.A second option would be to use another heater element in series with the one under test & measure the amperes, the reading will suddenly go up when the element fails because the circuit resistance will be cut in half.

author: ion
author_rank: Regular Contributor
timestamp: March 16, 2018, 07:22:10 pm
content: I'm glad it failed before I ordered the tester then!Two elements in series is a good idea, I'll keep it in mind in case I have this problem again. The hotplate uses 2 elements in parallel anyway, so it would be simple to rewire them in series and monitor the current.

author: Stray Electron
author_rank: Super Contributor
timestamp: March 18, 2018, 01:01:51 am
content: Quote from: ion on March 15, 2018, 01:47:02 pmI understand that's how the insulation test is normally done, but in this case I think the short was between the live and neutral, just before the element.That's the point in connecting live and neutral inputs together (and to one side of your meter). A short or even a partial short to frame ground (aka the chassis) on either line will show up on a resistance meter. Of course, your hot plate should be completely disconnected from the AC source at the time.If you wanted to get fancy you could monitor the current on the input, the neutral line and on the ground line. Once you see that the input and neutral currents are different then you KNOW that you have a problem. And any current flowing in the ground line confirms that.If you're relatively certain that it tripped the fuse and not the RCDand that you had the ground connectedthen you could have another problem somewhere else.

author: ion
author_rank: Regular Contributor
timestamp: March 19, 2018, 12:24:57 pm
content: Quote from: Stray Electron on March 18, 2018, 01:01:51 amI did test the continuity to earth and I had a separate plug-in RCD (to avoid tripping the RCD for the whole house). Neither RCD tripped so I'm sure it wasn't a fault to earth.It did however blow the fuses, and when I ran out of low current ones it started tripping the breaker. Once I removed the suspected faulty element I didn't have any more problems with the remaining one.I'm fairly certain the problem was a live to neutral short once the insulation broke down - somewhere between 30V and 240V. I can't really test it anymore as the wires broke while being removed from the hotplate.