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How do I rate a CT for leakage detection use? Also a DC leak detector coil?
741:
When there is no imbalance in L, N currents (no leakage) the CT sees no net current. For a major fault, current is large but presumably time-limited by MCB. If I have 230VAC/32A flowing in normal use, must a leakage detect CT be rated over 32A?
How is a DC leakage detect coil specified, is this a stock item? Basically I understand want to see measurable ferrite saturation, hence a change in inductance, at 6mA.
Gregg:
There are a number of stand alone ground fault sensor current transformers similar to this: https://www.nktechnologies.com/ground-fault-protection/
Eaton also makes them but their website sucks; here is an example on fleabay: https://www.ebay.com/itm/Eaton-Ground-Fault-Sensor-EGF3NCACNET3/283507373364 Some have adjustable threshold limits.
741:
Thank you:
We need to detect both AC (20mA) and DC (6mA) leakage, the latter relying of change of inductance with DC current. I wonder if the same coil can be calibrated to do double duty?
We do not have to measure normal-mode current with the coil, only leakage.
The spec is for an EV using AC charging, 32A and single or 3 phase (96A).
OpenEVSE use a 20A unit, 1000:1 ratio. Then 20mA leakage equates to just 20uA... :-\
duak:
741, are you aware of the principle behind the Flux-gate Magnetometer? In brief, a magnetic core is excited by a high frequency sine wave and the average current is monitored. The current to be measured is passed thru another winding. Because the core is magnetically non-linear, its inductance varies as a function of the inverse of the instantaneous current and so the average current is a facsimile of the current to be measured. These things can be quite sensitive, easily measuring mA. Practical units usually have two cores and separated excitation and sensing windings.
LEM makes current sensors, some of which use this principle. Here's a link to one of their products that explains it better: https://www.lem.com/en/file/5619/download
Since you want to measure leakage current, you'll want to run the line conductors, but not the earth or safety ground conductor through the core. This way, the operating currents cancel out leaving the leakage current to be measured. It should be possible to use a common mode choke with 2 or 3 identical windings for the line conductors and rated for the load current with an additional windings for excitation and sensing. Most common mode chokes use a fairly high mu material that has fairly sharp saturation characteristics.
Note, the excitation and sense circuitry should be designed to handle the surge energy passed thru the core during a fault.
741:
Thank you for the info aboux Flux-Gate methods.
The LEM diagrams show me how little I understand magnetics! Fig 5 is especially mysterious/impressive with the winding at RHS of continuously increasing diameter :popcorn:... LEM do say the FG approach can be made very accurate (more so than Hall effect). In my case I'm not quite so bothered about accuracy: I only want 'modest accuracy' leakage current measurement (5%?, eg 6mA +/- 0.3mA), WRT no leakage current of about zero.
I wonder if I can use Hall-effect in this application? One big issue for us is simply price - leakage detection looks set to use a fair proportion of our parts budget.
PS: What happens in a CT when the secondary current circuit is high resistance, do we see sparks as the PD rises to force the secondary current ratio relation? Or does the primary coil act to oppose the current in the conductor threaded through it? Presumably if both coils are High Z, nothing much happens and it's like there is no CT.
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