Medium Voltage Switchgear Megger Testing
Medium Voltage Switchgear Megger Testing
(OP)
When conducting a megger test of 4160V switchgear bus (cubicle to cubicle), how much resistance would a CT add to my result?
Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber






RE: Medium Voltage Switchgear Megger Testing
RE: Medium Voltage Switchgear Megger Testing
Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber
RE: Medium Voltage Switchgear Megger Testing
micro (or milli) ohms. If a Megger test was being performed with VT's (L-N connected primary), a very low value would be measured, through the VT winding itself. On a Megger, it may read very close to Zero.
As mentioned earlier, if the CT's are window type, they are not in physical connection with the bus bar. If the CT's are bar type, they would be included in the measurement. I'm fairly sure that a 70-80 micro ohm difference would not be picked up by a 1kV or 5kV Megger test set.
To directly answer the first question - A CT (bar type), would typically be expected to Lower the Insulation Resistance value slightly, depending on the condition of the CT an Switchgear.
RE: Medium Voltage Switchgear Megger Testing
A CT may make a small difference to the impedance with an AC ductor test.
The ductor may have been made by the Megger company and have a Megger logo.
Also, checking cubicle to cubical sounds more like a ductor test than a megger test.
Megger test are generally conducted from phase to phase or phase to ground rather fro cubicle to cubicle.
In very very old text books you may find a reference to a "wireless reactor".
This was a stack of iron laminations with a center hole. One of the secondary cables of a distribution transformer was passed through the hole in the iron laminations.
This would add some series impedance to facilitate the parallel operation of transformers with differing inpedances.
The core of the CT may act as a wireless reactor and add a small amount of impedance to the circuit being tested by a ductor.
Bill
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"Why not the best?"
Jimmy Carter
RE: Medium Voltage Switchgear Megger Testing
DTR2011's comment; "I'm fairly sure that a 70-80 micro ohm difference would not be picked up by a 1kV or 5kV Megger test set."
Most of the original meggers with a crank driven generator would not see a difference of 1000 Ohms.
The scales typically started at about 10,000 Ohms or 100,000 Ohms.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Medium Voltage Switchgear Megger Testing
Man is troubled by what might be called the Dog Wish, a strange and involved compulsion to be as happy and carefree as a dog --- James Thurber
RE: Medium Voltage Switchgear Megger Testing
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Medium Voltage Switchgear Megger Testing
RE: Medium Voltage Switchgear Megger Testing
Muthu
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