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DM7.ENG
Hi there,
Looking for some clarification on a call out I was sent to earlier today.
To set the scene
There is an MCC fed from a local transformer on site 11KVDY11 configuration.
I had a report from some of our electricians that whenattempting to obtain earth fault loop impedance values the meter would indicatea fault code. Usually this would indicate there is no presence of earth.
To add to matters it was reported that the phase to phasevoltages where 400V, Phase to neutral voltages where 230V however, the phase toPE voltages where something like L1-PE = 235V, L2-PE = 175V and L3-PE = 330V.
Straight away this suggested to me that there is an issuewith the incomer earth.
When I arrived on site I found that at the MCC incomer therewas 90Vac between earth and neutral and a current of 4A flowing in the mainearthing conductor.
The MCC consisted of a number of VSD’s, DOL motors and a fewlighting DB’s therefore, I isolated each load independently to record anydifference.
In the end I found that a mixer pump was causing thepotential difference between N&PE to rise from 19V to 90V when running. Icarried out an insulation resistance tested the mixer pump and found a deadshort between Phase and Earth.
The strange thing was the motor was operating regardless andthe 3 phase currents where balanced.
It then became clear that the reason the motor wouldcontinue to run even though it was down to earth was because there was no earththerefore, circuit protection would not operate because there is no faultcurrent path and hence no fault current.
I suspect that the earth electrode or earth tape at thetransformer is corroded or depredated due to electrolysis.
My question to help better understand this condition, whysuch voltage imbalance between Phase and earth when the defective motor isrunning.
I guess this is a prime example of what would happen iftransformer secondary’s are not grounded.
As always any advice is much appreciated.
Looking for some clarification on a call out I was sent to earlier today.
To set the scene
There is an MCC fed from a local transformer on site 11KVDY11 configuration.
I had a report from some of our electricians that whenattempting to obtain earth fault loop impedance values the meter would indicatea fault code. Usually this would indicate there is no presence of earth.
To add to matters it was reported that the phase to phasevoltages where 400V, Phase to neutral voltages where 230V however, the phase toPE voltages where something like L1-PE = 235V, L2-PE = 175V and L3-PE = 330V.
Straight away this suggested to me that there is an issuewith the incomer earth.
When I arrived on site I found that at the MCC incomer therewas 90Vac between earth and neutral and a current of 4A flowing in the mainearthing conductor.
The MCC consisted of a number of VSD’s, DOL motors and a fewlighting DB’s therefore, I isolated each load independently to record anydifference.
In the end I found that a mixer pump was causing thepotential difference between N&PE to rise from 19V to 90V when running. Icarried out an insulation resistance tested the mixer pump and found a deadshort between Phase and Earth.
The strange thing was the motor was operating regardless andthe 3 phase currents where balanced.
It then became clear that the reason the motor wouldcontinue to run even though it was down to earth was because there was no earththerefore, circuit protection would not operate because there is no faultcurrent path and hence no fault current.
I suspect that the earth electrode or earth tape at thetransformer is corroded or depredated due to electrolysis.
My question to help better understand this condition, whysuch voltage imbalance between Phase and earth when the defective motor isrunning.
I guess this is a prime example of what would happen iftransformer secondary’s are not grounded.
As always any advice is much appreciated.