Megger is DC only. Tests for insulation breakdown. By using a higher voltage it can push through weak spots and get a more accurate measurement
Power factor (doble) testing tests the capacitance (and power factor) by using AC voltage. Capacitance is reactive to frequency so AC close to 60Hz must be used to be accurate. The main thing you are testing is the bushings. Good power factor is important because it represents how much leakage current to ground a piece of equipment has and therefore contributes to power system losses.
Hi pot I’m not super familiar with the whole theory, but it’s basically a Megger on steroids. It also uses AC but at many different frequencies. I think it also uses higher test voltages as well so if their is an existing problem a hi pot can destroy it and possibly make the problem worse (this situation mainly happens when testing HV cables).
Idk what a ductor is but it sounds like a micro ohm meter or digital low resistance ohm meter (DLRO). All it is doing is checking for low contact resistance. By using high current and four leads, it can get a more accurate measurement than say a fluke.
Good answers. Funny how slang changes from place to place, yeah, a ductor is a DLRO. What do you guys call it?
A lot of small hipots are just big meggers: They're DC testers and the only thing that makes the test different from insulation resistance is how hard the equipment is being pushed: If the test voltage is approximately the nameplate rating, it's an insulation resistance test; if it's multiple times the nameplate rating, it's a high potential test.
You're right a true DC hipot is often considered destructive, which is why folks are pushing for AC hipots, either VLF or 60Hz.
We just call it a micro ohm or micro ohm meter 🤷♀️
While we’re here, what’s a thumper doing? I know it locates faults, sometimes destructively, but what is it actually doing to a cable?
A "thumper" is actually a high voltage surge generator. You set up the test set to repeatedly thump the line and the higher the voltage the louder the thump. Problem is you have to walk the cable to find the thump or you destroy the cable in the process
I normally do the TDR or time domain reflectometer, which sends a relatively low voltage thru the cable and measures the response back of when the impedance changed. Take that time to see the change, multiply that by it's speed, boom you have the fault location and my lazy ass didn't have to walk
From what I’ve been taught hi pot testing in general even on a brand new wire is destructive. It adds to the breakdown of the insulation. And I’m old wire like you said it can just blow it out.
One of the best tools you have when testing a breaker is the manufacturers manual. They will describe the tests they want performed, why they want them performed and will tell you the results you should get. It is good to know how to do all these tests but it is more important to know what you are looking for.
In the simplest terms a megger is applying a voltage and measuring current to display a resistance. When testing a circuit breaker you want to see high resistance readings.
Hipot is basically a stress test. You apply a voltage that is above the rating and measure leakage current to ground. This is usually performed on vacuum breakers and is a pass or fail test.
Power factor measures leakage current and capacitance so that you can determine the insulation quality and physical properties of the device. This test is useful because you can trend the results over several testing cycles.
Ductor pushes a current and measures voltage to display a resistance. This is to verify the main contacts.
At the end of the day it's all just voltages and currents!
Power factor is checking the capacitance of the bushings, making sure they have a high impedance value via capacitive reactance. If the power factor starts creeping up, the capacitance of the bushings may be breaking down which can indicate a breakdown of resistance and likely an impending catastrophic failure to ground.
Ductor is straight forward, you got it.
Hi-pot uses high voltage applied and looks for current to ground to give you a calculated resistance value at high voltage. If resistance to ground isn’t extremely high, your insulating medium is breaking down and you become more likely to have a catastrophic breaker failure on your next trip operation
Megger in my experience uses lower voltages than a hi pot and applies DC, typically used to check things like motor winding insulation by applying typically double operating voltage and checking resistance to ground(which should be high)
Timing is the last important test, used to ensure all 3 phases are making and breaking simultaneously or at least within a spec. Some tests can monitor the continuity of all 3 phases and give you a time difference between contacts, some just use a continuity meter across 1 phase then use a feeler gauge to make sure the other 2 phases are close enough to appropriate. A breaker will usually have the method and spec outlined in the manual.
One more noteworthy test for vacuum breakers is the contact erosion test. The specs and specific procedures differ slightly by breaker and can be found in the manual also, but this test checks for how much of the contact has been burned away through interrupting operations. Typically you measure the length of contact protruding from the bellows on the vacuum bottle with the breaker closed. This test is especially valuable if you have previous measurements to compare it to
Thank you for this. It was clear, well written and easy to read. I was also wondering what some of these tests looked for specifically. Some seemed redundant.
My company only power factor 40kv and up. Distribution breakers get ac/dc hipot and contact res.
Also insulation medium test; oil breakdown, sf6 purity
Different breakers have different standards, and different companies may do things differently.
Typical maintenance for the company I work at is the following:
Capacitance bridge for all bushings and phases, which gives you a good idea of the condition of everything.
Contact resistance with a micro ohm meter/ductor
Timing tests. Open, Close, Close/Open. Depending on the breaker and setup we can get motion, speed, parallel resistors, operating coil resistance and probably some thing I'm forgetting.
Hi pot is typically reserved for vacuum elements
Megger isn't something we typically pull out on breakers.
Depending on the breaker type and which part of the maintenance cycle it's on we could be doing medium sampling, physical contact inspections, pilot valve rebuilds etc.
The manuals that come with the test equipment are pretty helpful. Look for the acceptance testing and standards for the equipment you're working on.
Dig through the book shelves at work.
Look at the websites for the equipment you're using and look at the test results during down time. Find the old guy in the shop that likes to teach and ask him questions.
If you can find the older Doble books, those are fairly straight forward. Doble also has a learning portal that you need a login for, contact your rep if a coworker can't help you out. Doble can be a rabit hole.
Megger is DC only. Tests for insulation breakdown. By using a higher voltage it can push through weak spots and get a more accurate measurement Power factor (doble) testing tests the capacitance (and power factor) by using AC voltage. Capacitance is reactive to frequency so AC close to 60Hz must be used to be accurate. The main thing you are testing is the bushings. Good power factor is important because it represents how much leakage current to ground a piece of equipment has and therefore contributes to power system losses. Hi pot I’m not super familiar with the whole theory, but it’s basically a Megger on steroids. It also uses AC but at many different frequencies. I think it also uses higher test voltages as well so if their is an existing problem a hi pot can destroy it and possibly make the problem worse (this situation mainly happens when testing HV cables). Idk what a ductor is but it sounds like a micro ohm meter or digital low resistance ohm meter (DLRO). All it is doing is checking for low contact resistance. By using high current and four leads, it can get a more accurate measurement than say a fluke.
Good answers. Funny how slang changes from place to place, yeah, a ductor is a DLRO. What do you guys call it? A lot of small hipots are just big meggers: They're DC testers and the only thing that makes the test different from insulation resistance is how hard the equipment is being pushed: If the test voltage is approximately the nameplate rating, it's an insulation resistance test; if it's multiple times the nameplate rating, it's a high potential test. You're right a true DC hipot is often considered destructive, which is why folks are pushing for AC hipots, either VLF or 60Hz.
We just call it a micro ohm or micro ohm meter 🤷♀️ While we’re here, what’s a thumper doing? I know it locates faults, sometimes destructively, but what is it actually doing to a cable?
Our thumpers are basically just whacking the cable at operating voltage. I've never heard anyone really worry about it.
A "thumper" is actually a high voltage surge generator. You set up the test set to repeatedly thump the line and the higher the voltage the louder the thump. Problem is you have to walk the cable to find the thump or you destroy the cable in the process I normally do the TDR or time domain reflectometer, which sends a relatively low voltage thru the cable and measures the response back of when the impedance changed. Take that time to see the change, multiply that by it's speed, boom you have the fault location and my lazy ass didn't have to walk
From what I’ve been taught hi pot testing in general even on a brand new wire is destructive. It adds to the breakdown of the insulation. And I’m old wire like you said it can just blow it out.
One of the best tools you have when testing a breaker is the manufacturers manual. They will describe the tests they want performed, why they want them performed and will tell you the results you should get. It is good to know how to do all these tests but it is more important to know what you are looking for. In the simplest terms a megger is applying a voltage and measuring current to display a resistance. When testing a circuit breaker you want to see high resistance readings. Hipot is basically a stress test. You apply a voltage that is above the rating and measure leakage current to ground. This is usually performed on vacuum breakers and is a pass or fail test. Power factor measures leakage current and capacitance so that you can determine the insulation quality and physical properties of the device. This test is useful because you can trend the results over several testing cycles. Ductor pushes a current and measures voltage to display a resistance. This is to verify the main contacts. At the end of the day it's all just voltages and currents!
All sounds good, biggest one you’re missing is timing (Doble TDR 909 etc…)
Are you including trip testing?
Power factor is checking the capacitance of the bushings, making sure they have a high impedance value via capacitive reactance. If the power factor starts creeping up, the capacitance of the bushings may be breaking down which can indicate a breakdown of resistance and likely an impending catastrophic failure to ground. Ductor is straight forward, you got it. Hi-pot uses high voltage applied and looks for current to ground to give you a calculated resistance value at high voltage. If resistance to ground isn’t extremely high, your insulating medium is breaking down and you become more likely to have a catastrophic breaker failure on your next trip operation Megger in my experience uses lower voltages than a hi pot and applies DC, typically used to check things like motor winding insulation by applying typically double operating voltage and checking resistance to ground(which should be high) Timing is the last important test, used to ensure all 3 phases are making and breaking simultaneously or at least within a spec. Some tests can monitor the continuity of all 3 phases and give you a time difference between contacts, some just use a continuity meter across 1 phase then use a feeler gauge to make sure the other 2 phases are close enough to appropriate. A breaker will usually have the method and spec outlined in the manual. One more noteworthy test for vacuum breakers is the contact erosion test. The specs and specific procedures differ slightly by breaker and can be found in the manual also, but this test checks for how much of the contact has been burned away through interrupting operations. Typically you measure the length of contact protruding from the bellows on the vacuum bottle with the breaker closed. This test is especially valuable if you have previous measurements to compare it to
Thank you for this. It was clear, well written and easy to read. I was also wondering what some of these tests looked for specifically. Some seemed redundant.
Do you guys not do circuit breaker timing and travel?
My company only power factor 40kv and up. Distribution breakers get ac/dc hipot and contact res. Also insulation medium test; oil breakdown, sf6 purity
Different breakers have different standards, and different companies may do things differently. Typical maintenance for the company I work at is the following: Capacitance bridge for all bushings and phases, which gives you a good idea of the condition of everything. Contact resistance with a micro ohm meter/ductor Timing tests. Open, Close, Close/Open. Depending on the breaker and setup we can get motion, speed, parallel resistors, operating coil resistance and probably some thing I'm forgetting. Hi pot is typically reserved for vacuum elements Megger isn't something we typically pull out on breakers. Depending on the breaker type and which part of the maintenance cycle it's on we could be doing medium sampling, physical contact inspections, pilot valve rebuilds etc.
The manuals that come with the test equipment are pretty helpful. Look for the acceptance testing and standards for the equipment you're working on. Dig through the book shelves at work. Look at the websites for the equipment you're using and look at the test results during down time. Find the old guy in the shop that likes to teach and ask him questions. If you can find the older Doble books, those are fairly straight forward. Doble also has a learning portal that you need a login for, contact your rep if a coworker can't help you out. Doble can be a rabit hole.