If you think that Uni-T 135C is bad (and I agree with you), just be glad you haven't had the misfortune to encounter one of these turds:
Yes, there are high resolution pictures of the top and bottom sides of the UT125C PCB on HKJ's website.
If that happens, arc would exit the case following the cables.
It might.... if you used it outside the CAT III environment that it's rated for.
And where do you think CAT III is?
On the safest side of a distribution panel with circuit breakers in it.
We still don't know for sure if what we're looking at is a spark gap or where it is in the meter's input circuit, eg. Is it in series with the fuse? To me it looks like there could be a current shunt across that gap in some variant of the mater (between the two vias where the vertical white line is painted).
If that happens, arc would exit the case following the cables.
It might.... if you used it outside the CAT III environment that it's rated for.
Yes, there are high resolution pictures of the top and bottom sides of the UT125C PCB on HKJ's website.
Can you pop the fuse out and confirm which end the positive lead is connected to?
Edit: it appears from one of HKJ's other photos that it is connected to the top (near the PTCs) as one would hope.
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We still don't know for sure if what we're looking at is a spark gap or where it is in the meter's input circuit, eg. Is it in series with the fuse?
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... (insulation is badly damaged on cables on that photo..) ...
Yes, the positive lead is connected to the side of the fuse that meets the PTCs. So I would guess if ever there is a transient that arcs across the gap, it should blow the fuse?
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So here is a plausible example. Suppose you have a large machine that uses 10 of these motors:
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A phase-to-phase fault on a 480-V system with 20,000 amperes of fault current provides 9600000 watts of power (9,6 MW).
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Yes, there are high resolution pictures of the top and bottom sides of the UT125C PCB on HKJ's website.
Can you pop the fuse out and confirm which end the positive lead is connected to?
Edit: it appears from one of HKJ's other photos that it is connected to the top (near the PTCs) as one would hope.
Yes, the positive lead is connected to the side of the fuse that meets the PTCs. So I would guess if ever there is a transient that arcs across the gap, it should blow the fuse?
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So here is a plausible example. Suppose you have a large machine that uses 10 of these motors:
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A phase-to-phase fault on a 480-V system with 20,000 amperes of fault current provides 9600000 watts of power (9,6 MW).
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Excluding the apocalyptic scenarios, we are talking about a $15 10cm tall DMM for home use, certainly not an electrician's tool for use in industrial settings. All I am interested in is that it is independently certified to a CAT III 600V rating. And of course I am curious about the spark gap. Btw if you look closely it also has a PCB cutout under the (beefy?) PTCs. So it seems that UNI-T spent a few extra cents to try to meet the CAT III rating, but was the effort and money well spent?
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Yes, the positive lead is connected to the side of the fuse that meets the PTCs. So I would guess if ever there is a transient that arcs across the gap, it should blow the fuse?It will blow the fuse, but they are slow devices. ...
All I am interested in is that it is independently certified to a CAT III 600V rating.
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Yes, the positive lead is connected to the side of the fuse that meets the PTCs. So I would guess if ever there is a transient that arcs across the gap, it should blow the fuse?It will blow the fuse, but they are slow devices. ...
I believe there is a PTC+MOV in parallel with the spark gap + fuse across the input leads, shouldn't that react faster than the fuse?
So the transient energy would be absorbed in two steps, first by the PTC+MOV combo and then eventually second, by arcing across the spark gap and blowing the fuse?
I haven't measured the width of the spark gap but it seems to be about 1mm to 1.5mm wide, so the transient would need to be above 3kV to 4.5kV to arc across it, or am I getting it wrong?
I believe there is a PTC+MOV in parallel with the spark gap + fuse across the input leads, shouldn't that react faster than the fuse?
I haven't measured the width of the spark gap but it seems to be about 1mm to 1.5mm wide, so the transient would need to be above 3kV to 4.5kV to arc across it, or am I getting it wrong?
Btw if you look closely it also has a PCB cutout under the (beefy?) PTCs.
...., but Germans being traditionally thorough, ....
A phase-to-phase fault on a 480-V system with 20,000 amperes of fault current provides 9600000 watts of power (9,6 MW). If the fault lasts for 200 milliseconds before the overcurrent protection clears it, the released energy would be 1,92 MJ, which corresponds roughly to a stick of dynamite.
if it turns out that it got that CAT III/600V rating through entirely legitimate means, then the CAT ratings themselves are not a very strong indication of anything. And that is bad for everyone, IMO.
With arc accidents, it is not transient that does damage. Transient creates plasma that creates short fault.
And power grid powers the explosion
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As I stated, there's nothing there to save the PTCs except the PCB and leads. Maybe the MOVs would open up first.
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Those surge rated resistors are expensive. Lets just put two 5mm PTCs in series. What could go wrong?
IIRC the CATIII 600V rating means the DUT has to withstand 10 repeated transients, so if the PTC fails catastrophically after the first or second transient, the DUT does not get the rating?
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As I stated, there's nothing there to save the PTCs except the PCB and leads. Maybe the MOVs would open up first.
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Those surge rated resistors are expensive. Lets just put two 5mm PTCs in series. What could go wrong?
I think I understand: a surge rated resistor is needed in series with the PTC, to absorb part of the energy of the transient until the PTC has enough time to heat up and see its resistance value increase. If there is no surge rated resistor, the PTC absorbs all the energy of the transient before its resistance value has enough time to increase and could eventually fail catastrophically.
IIRC the CATIII 600V rating means the DUT has to withstand 10 repeated transients, so if the PTC fails catastrophically after the first or second transient, the DUT does not get the rating?