In distribution subs they have thermal ammeters I think they have a 30 minute warm up time so they don't get hammered by large starting currents or faults. I think I'm right in saying that the meters are switched out if circuit forr most of the time I'm you'd you can't leave CTs open circuited so do they just put a resistor across the CTs in are the meters usually left in circuit.
Also worth considering that, without a resistor or short at the current transformer, the voltage will not be maintained in the SELV range.
You can buy current transformers that are guaranteed SELV, but they are very expensive.
For other current transformers, these need to be treated as FELV at least, if not LV, and preferably go in an enclosure. Similarly, insulated wires not in containment or sheath wouldn't meet BS 7671 requirements.
Only mentioning this, as current transformer installation is ramping up with PV inverters and EV charging equipment ... but not always installed according to BS 7671 requirements because the actual potential voltage range is not considered.
Needs a double plus good marking for that 
Also a lot of "consumer" CTs have either a burden resistor or voltage-clamping (e.g. back to back zeners) built-in.
- Andy.
Agree, and to clarify we mean anti-series Avalanche or Zener diodes, not anti-parallel. You can get some nice ones you can plug into a cheap PC sound "card" with good LF response with just the burden resistor added, that allow you to look at waveform qualityand harmonic analysis.
the insulation from the meter tails is not the problem,
No, it's the insulation and protection of the conductors from the current transformer, and of the current transformer itself, because of the voltages created. We no longer have 'limitation of discharge of energy', and there are good reasons for that.
These conductors are not SELV ... so they must be FELV or LV ... and you see where this goes from here in BS 7671 terms.
Also a lot of "consumer" CTs have either a burden resistor or voltage-clamping (e.g. back to back zeners) built-in.
I'm not 100 % sure this is the case ... and if it is, what if those devices fail? 'Guaranteed SELV' CTs are very expensive!
One example: https://www.makerhero.com/img/files/download/SCT013-Datasheet.pdf
Failure of the voltage clamping must be possible of course.It would have to be part of a chain of failures to become hazardous though - e.g. circuit broken circuit plus failed basic insulation, or being dismantled/disconnected without taking the basic precaution of unclipping it from the meter tails as well as the monitored circuit carrying a significant current at the moment concerned - combined with the chances of the voltage clamping itself failing. So maybe not drastically more dangerous than say ADS where the simple failure of a c.p.c. can entirely remove protection.
The high voltage is associated with an infinite impedance of an open circuit though - to get a shock presumably a body resistance (in the region of 1kΩ?) would have to be in-circuit and therefore limit the voltage (and current) available to some extent?
- Andy.
Failure of the voltage clamping must be possible of course.It would have to be part of a chain of failures to become hazardous though - e.g. circuit broken circuit plus failed basic insulation,
The device you linked to has an audio jack on it, so only one fault (failed resistor) is necessary.
However, that is also based on the assumption that the burden resistor type is selected. The 1 V output is pretty non-standard for a monitoring interface in a building, because there could well be a problem with EMC if there's a long-lead connection in an installation.
I've certainly seen a lot that are a basic current transformer, no built-in burden resistor, with simple insulation on the leads for connection - these only need a single fault (open-circuit conductor) to be a hazard.
Unfortunately, that ship is already passing through the harbour entrance and heading out into the world. Consumer CTs intended to clip round meter tails with whatever protection their suppliers deem appropriate have been out there for over 20 years (at least since the first home energy monitors appeared in the 1990s) and the fitment rate is taking off rapidly with the proliferation of EVSE and PV. Some of them make a nice buzzing sound if clipped round a tail without being connected - I'm not sure whether that is internal arcing or the operation of a protective device and haven't had the time to investigate.
Ideally we would have rolled out a simple standard interface giving basic electrical data (voltage, current, power, etc.) as part of the smart meter roll-out, completely avoiding the need for clip on CTs (a great application for cheap plastic fibre optics, since only uni-directional communication over relatively short distances is required).
(I also note that smart meters are probably the logical place for things such as a standardised approach to G100 style CLSs (especially in a domestic setting and if multiple manufacturers devices need to work together) and possibly even open PME neutral detection, but maybe I'm getting a little controversial now...).
Ideally we would have rolled out a simple standard interface giving basic electrical data (voltage, current, power, etc.) as part of the smart meter roll-out, completely avoiding the need for clip on CTs (a great application for cheap plastic fibre optics, since only uni-directional communication over relatively short distances is required).
For energy management, it's important to be able to look at import and export current as well as voltage.
There are alternative approaches, where an energy monitoring unit (that has CTs and VTs built in) connects in-line with the tails, and communicates via CANbus/Modus back to the energy management system (e.g. inverter).
Consumer CTs intended to clip round meter tails with whatever protection their suppliers deem appropriate have been out there for over 20 years (at least since the first home energy monitors appeared in the 1990s) and the fitment rate is taking off rapidly with the proliferation of EVSE and PV.
Agreed ... it's those I'm talking about, not all are installed correctly (and as discussed above, perhaps some are not installed in accordance with BS 7671).
The device you linked to has an audio jack on it, so only one fault (failed resistor) is necessary.
Fair point. Of course wire-ended versions are available too. Interestingly Victron seem to have switched from plugs to hard-wired in their recent models - they might have had similar thoughts.
- Andy.
The device you linked to has an audio jack on it, so only one fault (failed resistor) is necessary.
Fair point. Of course wire-ended versions are available too. Interestingly Victron seem to have switched from plugs to hard-wired in their recent models - they might have had similar thoughts.
- Andy.
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