I posted this before here I THINK, but if you are going to megger busway you should have an understanding of whats involved.
There are a variety of test that can be performed with meggers, but the primary test most electricians utilize is a go no go test, connect the megger record the readings and make a determination if the readings are in an acceptable range
Busway:
Busways should be meggered piece by piece as it comes off the truck. Then piece by piece as it goes up, the assembled busway should be meggered as each piece is added and lastly the completed busway should be meggered.
Feeders:
Feeders should be meggered as soon as they are pulled and after termination.
Switchboards and MCCs:
Switchboards should be meggered as soon as they arrive on site, after installation and prior to energizing. In addition The neutral disconnect link should be removed and the downstream neutral meggered to ground, this verifies the downstream neutral is clear from shorts to ground avoiding net current issues and the resulting EMF, additionally should you lose the neutral connection to the main switchboard the downstream connections will carry the full neutral load, which can be a fire hazard.
Panels:
Panels should be meggered when they are installed and prior to energizing. The branch circuit neutrals should be isolated from the feeder neutral and meggered to verify there no branch circuit neutral shorts to ground. AVOID NET CURRENTS and EMF
Transformers:
Like all equipment it should be meggered prior to energizing. You can not megger phase to phase as this is a direct connection through the winding. Megger phase to ground on the Primary, Phase to ground on the secondary (with the neutral ground bond lifted) and primary to secondary.
Circuit Breakers:
Large frame circuit breakers should be meggered in particular after a fault when the CB has operated. Megger line to load across each phase, phase to phase line and load and phase to ground. A simple method to do these test is to open the CB, place jumpers from Line B phase to A and C phase load side then jumpers from B phase load to line side A and C. Place your megger leads on B line and B load record your readings> No close the CB and megger to ground.
ATS Automatic Transfer Switches:
Care should be taken with ATS’s the control panel must be disconnected to avoid damaging the control panel.
Motor:
Motors should be meggered prior to energizing, remember that phase to phase SHOULD read a dead short you only need to megger to ground.
Obviously and cables, busway, switchboards, motors and any distribution or utilization equipment suspected of having a short should be meggered.
With Electrical Preventative Maintenance (EPM), large distribution equipment should be meggered prior to and after service. Should there be a drastic change in readings you will realize you have possible made an error in your service or if the switch board has an existing problem you avoid being blamed for affecting the integrity of the switchboard. I am sure all of us have heard at least once “It worked when you got here”.
Prior to megging at a high voltage it is wise to either megger at 100 VDC or use an ohm meter to verify there are no loads on the equipment to be meggered, Protective relays, metering hidden bus taps, or customer utilization loads. If you obtain a reading LOCATE the load, sometimes this takes some perseverance.
What you should see:
We like to see full scale readings, 2000 megohms use to be the standard for most meggers but now meggers come with any were from 1000 megohms to 1000 gigohms full scale.
A simple test is to turn the megger and grab the test leads (NO JUST SEEING IF I HAVE BORED YOU TO SLEEP). With the megger on leads separate push the test button, air is an insulator and the reading should be full scale, touch the leads dead short megger should read zero (0). Take a piece of paper the thinner the better, using a pencil make a scratchy drawing all over with two heavy squares on the edge about 4” apart. Connect your megger and megger the paper at 1000 VDC, you should see arcing and may set the paper on fire or the arcing may burn the short free.
While some standards list .5 megohms (500,000 ohms) as acceptable, we like to see at least 5 megohms (5,000,000 ohms), 50 megohms is preferable (50,000,000 ohms) But if this is new equipment one would expect to see full scale readings, often refered to as infinity. It really not infinity your meter just does not have high enough resolution.
If the reading on A phase to ground is 50 megohms and B and C phase are 1000 megohms, the A phase should be checked for issues. With all things the same Temperature, humidity, age of equipment, and environment (dirt dust ECT) readings should be close.
Readings are taken A to B, B to C, C to A, and A, B, C and sometimes neutral to ground.
It is also a common practice to ground the phases not under test. Except motors and transformers.
Long cable runs and busway may take a while for the test voltage to charge the conductors under test. The conductors are a large capacitor and when the readings stabilize and max out the readings is recorded. This conductor MUST BE DISCHARGED to ground after test unless you want to receive a nasty shock.
Megger readings that do not stabilize, but fluctuate are indicative of moisture in or on the conductors and should be investigated. Sometimes this reading will continue to climb as the moisture evaporates due to the test voltage.
When meggering one must be careful with the voltage levels you are trying to prevent future electrical damage to cause it.
I use 5 different meggers, two are pictured below. All the meggers we use are digital some with an analog scale. We calibrate all our test equipment and meters yearly.
The f1st megger is a simple compact 1000 VDC go, no go megger for day to day use.
The 2nd megger I use has 50, 100, 250, 500 and 1000 VDC. I like this meter because of the 50 and 100 VDC settings.
The 3rd megger I use a hand crank model, I keep this on my truck for emergencies (LIKE DEAD BATTERIES).
The 4th megger is a 5KV (5000 VDC) with 1000, 2500 and 5000 settings, this meter is a battery and 120 VAC powered, the advantage is this meter hasa run timer for different test that require time test. Some spec. jobs require a one minute test for feeders.
The 5th megger is a ESI (Electric Static) tester with 10 and 100 VDC settings and a built in hygrometer and thermometer.
The last meter shown is a thermometer/hygrometer for measuring temperature and humidity
There are a variety of test that can be performed with meggers, but the primary test most electricians utilize is a go no go test, connect the megger record the readings and make a determination if the readings are in an acceptable range
Busway:
Busways should be meggered piece by piece as it comes off the truck. Then piece by piece as it goes up, the assembled busway should be meggered as each piece is added and lastly the completed busway should be meggered.
Feeders:
Feeders should be meggered as soon as they are pulled and after termination.
Switchboards and MCCs:
Switchboards should be meggered as soon as they arrive on site, after installation and prior to energizing. In addition The neutral disconnect link should be removed and the downstream neutral meggered to ground, this verifies the downstream neutral is clear from shorts to ground avoiding net current issues and the resulting EMF, additionally should you lose the neutral connection to the main switchboard the downstream connections will carry the full neutral load, which can be a fire hazard.
Panels:
Panels should be meggered when they are installed and prior to energizing. The branch circuit neutrals should be isolated from the feeder neutral and meggered to verify there no branch circuit neutral shorts to ground. AVOID NET CURRENTS and EMF
Transformers:
Like all equipment it should be meggered prior to energizing. You can not megger phase to phase as this is a direct connection through the winding. Megger phase to ground on the Primary, Phase to ground on the secondary (with the neutral ground bond lifted) and primary to secondary.
Circuit Breakers:
Large frame circuit breakers should be meggered in particular after a fault when the CB has operated. Megger line to load across each phase, phase to phase line and load and phase to ground. A simple method to do these test is to open the CB, place jumpers from Line B phase to A and C phase load side then jumpers from B phase load to line side A and C. Place your megger leads on B line and B load record your readings> No close the CB and megger to ground.
ATS Automatic Transfer Switches:
Care should be taken with ATS’s the control panel must be disconnected to avoid damaging the control panel.
Motor:
Motors should be meggered prior to energizing, remember that phase to phase SHOULD read a dead short you only need to megger to ground.
Obviously and cables, busway, switchboards, motors and any distribution or utilization equipment suspected of having a short should be meggered.
With Electrical Preventative Maintenance (EPM), large distribution equipment should be meggered prior to and after service. Should there be a drastic change in readings you will realize you have possible made an error in your service or if the switch board has an existing problem you avoid being blamed for affecting the integrity of the switchboard. I am sure all of us have heard at least once “It worked when you got here”.
Prior to megging at a high voltage it is wise to either megger at 100 VDC or use an ohm meter to verify there are no loads on the equipment to be meggered, Protective relays, metering hidden bus taps, or customer utilization loads. If you obtain a reading LOCATE the load, sometimes this takes some perseverance.
What you should see:
We like to see full scale readings, 2000 megohms use to be the standard for most meggers but now meggers come with any were from 1000 megohms to 1000 gigohms full scale.
A simple test is to turn the megger and grab the test leads (NO JUST SEEING IF I HAVE BORED YOU TO SLEEP). With the megger on leads separate push the test button, air is an insulator and the reading should be full scale, touch the leads dead short megger should read zero (0). Take a piece of paper the thinner the better, using a pencil make a scratchy drawing all over with two heavy squares on the edge about 4” apart. Connect your megger and megger the paper at 1000 VDC, you should see arcing and may set the paper on fire or the arcing may burn the short free.
While some standards list .5 megohms (500,000 ohms) as acceptable, we like to see at least 5 megohms (5,000,000 ohms), 50 megohms is preferable (50,000,000 ohms) But if this is new equipment one would expect to see full scale readings, often refered to as infinity. It really not infinity your meter just does not have high enough resolution.
If the reading on A phase to ground is 50 megohms and B and C phase are 1000 megohms, the A phase should be checked for issues. With all things the same Temperature, humidity, age of equipment, and environment (dirt dust ECT) readings should be close.
Readings are taken A to B, B to C, C to A, and A, B, C and sometimes neutral to ground.
It is also a common practice to ground the phases not under test. Except motors and transformers.
Long cable runs and busway may take a while for the test voltage to charge the conductors under test. The conductors are a large capacitor and when the readings stabilize and max out the readings is recorded. This conductor MUST BE DISCHARGED to ground after test unless you want to receive a nasty shock.
Megger readings that do not stabilize, but fluctuate are indicative of moisture in or on the conductors and should be investigated. Sometimes this reading will continue to climb as the moisture evaporates due to the test voltage.
When meggering one must be careful with the voltage levels you are trying to prevent future electrical damage to cause it.
I use 5 different meggers, two are pictured below. All the meggers we use are digital some with an analog scale. We calibrate all our test equipment and meters yearly.
The f1st megger is a simple compact 1000 VDC go, no go megger for day to day use.
The 2nd megger I use has 50, 100, 250, 500 and 1000 VDC. I like this meter because of the 50 and 100 VDC settings.
The 3rd megger I use a hand crank model, I keep this on my truck for emergencies (LIKE DEAD BATTERIES).
The 4th megger is a 5KV (5000 VDC) with 1000, 2500 and 5000 settings, this meter is a battery and 120 VAC powered, the advantage is this meter hasa run timer for different test that require time test. Some spec. jobs require a one minute test for feeders.
The 5th megger is a ESI (Electric Static) tester with 10 and 100 VDC settings and a built in hygrometer and thermometer.
The last meter shown is a thermometer/hygrometer for measuring temperature and humidity
