Electronics > Projects, Designs, and Technical Stuff
How do I rate a CT for leakage detection use? Also a DC leak detector coil?
uer166:
--- Quote from: NiHaoMike on February 14, 2020, 12:20:17 am ---I'm having a hard time thinking of a realistic fault with an EV that would cause a ground fault not involving the chassis.
--- End quote ---
That is irrelevant, it will fail all (almost all) IEC 62955 tests that you have to do to certify the RDC-DD 6mA detection scheme. The entire point of all these leakage detectors is to protect people/equipment when the isolation in the car fails, if it doesn't work in that scenario then what's the point?
Here's a fault not involving the chassis: a person standing on the ground touches a live conductor of the EVSE cord due to insulation failure and gets a shock. The return path is the earth they're standing on. A correctly designed leakage detector will pick that up, anything involving monitoring the GND path in the EVSE will not.
--- Quote ---All the high voltage wiring is well protected for a good reason.
--- End quote ---
If that was true there would be no need for any leakage detectors ever, and no need for double insulated DC chargers as well, yet all EVSE in europe have both AC and DC leakage detection, and all DC chargers have a double insulated HF transformer. But again all this is irrelevant, any standards body will immediately fail that design without even testing it.
max_torque:
A lot of the EVSE monitoring capability is mandated to protect against upstream failures (ie the supply side) rather than with the vehicle itself!
https://professional-electrician.com/technical/ev-charging-equipment-the-new-rules-explained/
The choice between developing your own fault current detection systems and buying an OTS component really fall to the choice between cost and liability. When you buy an OTS solution, you carry across the certifications of that solution, so yes, it costs more, but you have some legal protections. If you develop your own solution, then you entirely "own" the liabilities for that solution.
So, if you're going to sell hundreds of thousands,then rolling your own looks sensible, as you can afford to amortise the test and certification for your device over many more units, but if you are only makes hundreds or low thousands, then an OTS solution may be a better idea overall
NiHaoMike:
--- Quote from: uer166 on February 14, 2020, 05:30:29 am ---Here's a fault not involving the chassis: a person standing on the ground touches a live conductor of the EVSE cord due to insulation failure and gets a shock. The return path is the earth they're standing on. A correctly designed leakage detector will pick that up, anything involving monitoring the GND path in the EVSE will not.
--- End quote ---
That would get picked up by the AC RCD.
741:
The video teardown in post #16 is narrated by two UK electricians. I took notes, this is what struck me as important -
(1) "OLEV" - A UK grant scheme from the "Office for Low Emission Vehicles". User wants compatibility. A new requirement of the scheme, is that "as from July 2019 EVSE MUST be able to connect to the internet".
(2) TN-C-S is the most common earthing arrangement in the UK //See also https://medium.com/@Voltimum/the-principles-of-protective-multiple-earthing-pme-c068f2f433ac
Wiring Regulations (18th edition, section 722) FORBID TN-C-S for EVSE outdoor stations - other than for three specific exceptions.
"A PME earthing facility shall not be used as the means of earthing" //Wiring regulations, 18th ed regs, part 722
However, TN-C-S is an example of a PME system.
https://www.voltimum.co.uk/articles/do-you-need-install-earth-rod-ev-charge
https://eocharging.zendesk.com/hc/en-us/articles/360024737931-18th-Edition-Wiring-Regulations
Normal practice for outdoor EVSE is to add an earth rod, creating a local 'TT' system. Zappi 2 uses an exception to the TN-C-S rule to save the need for an earth rod.
My understanding is TN-C-S has the following features:
(a) From the source supply and up to the supplier fuse, Earth & Neutral are one single conductor ('Terre' & Neutral are Combined hence 'TN-C'). 'Terre' = 'Earth'.
(b) Along the route of this "Combined" conductor, there are "multiple earths". So TN-C-S is a 'PME' system, "Protective Multiple Earth". NOTE: The actual combined Earth/Neutral conductor itself is termed 'PEN conductor' or 'CNE conductor'.
Why is TN-C-S normally forbidden for outdoor EVSE? It is to allow for the case where the supplier's PME combined conductor (called 'PEN' or 'CNE') breaks outside your property. The current return path for both 'line to earth' faults and line to neutral (fault or normal use) is via the combined PEN conductor.
But if the PEN breaks, then even normal-use Neutral flow will go the easiest way to 'Terre' (Earth). Maybe a person is touching the car. The current cannot return via the failed PEN cable though. So it will opportunistically jump via the person into the floor that person is standing on.
* Question: Why is there no requiremnet for a house-local earthing rod in all installations, regardless of outdoor equipment? - the same scenario can occur indoors too.
Of the 3 exceptions mentioned above, two are pretty rare situations. The 3rd exception allows TN-C-S if the safety RCD cuts "Earth" as well as Live. Zappi 2 uses this method, hence saving the installer worrying how good a local earth they can create, especially regarding dry soil. The narrators like this feature, apparently MyEnergi/Notts University have notes explaining how they do this. It would seem to be basically the CT around the earth lead that trips the 'contactor' in the earth line.
(3) Zappi 2 provides energy mangement. Part of this relies on up to 3 external Current Transfomer inputs (note these can be direct wired or more conveniently the current value can be sent by RF link).
The RF link is "NOT wifi or Bluetooth" for reliability reasons. There is also a CAT5 socket. Unsure if the RF link still used then for CT, probably.
There is a danger you can blow the main fuse if other high current equipment is also running. Zappi 2 monitors whole system via CTs [+RF link] etc to ensure no main fuse overload. For instance
CT 1 on main supply to house //Main fuse eg 100A
CT 2 on cable to garage //MCB rating to garage eg 40A "sub-main line conductor"
Then program Zappi 2 to respect those limits //Is this via a page on an HTML server inside Zappi 2?
* Question: Assuming it is a real pain to replace a blown supplier's fuse, why is there not usually a high-current MCB immediately downstream from the supplier's fuse, thus protecting the supplier's fuse? Eg Maybe something just under the main fuse rating?
(4) When contact weld seen, the PCB mounted DPST relay just up from the LNE input connector disconnects L & E.
(5) The narrators say Zappi 2 incorporates a class A RCD into its PCB circuitry and a CT. Further, the same CT (with L, N through it) also monitors for DC current over 6mA.
(6) There is also a CT around Live, presumably they measure current draw this way.
(7) We know front right is a PCB mounted relay. Not sure what the 2 large black boxes at rear are. Rear centre looks like a transformer. Rear right looks like a relay.
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