Hioki makes good meters but the early teardowns on this forum are puzzling.
Its been several years since earlier DMMs in that Hioki series were released and I still cannot find a mention of any NRTL safety testing mark beyond Hioki's own "CE." Does anyone here know it exists?
Back in 2013, a teardown was done and this is how the DT4252 input stage appears, the COM [center terminal] and 10A input [ lower right] traces are a bit close for creepage comfort.
The designers were aware of the risk and gave due creepage and clearance of V input [ left most jack] from commons, center terminal.
You can easily follow the tracing to the fuse, and note the other end of the fuse is close to the 10A input jack and the tracings are very close to each other, almost shorting across the fuse.
Granted, this is a low impedance path if the fuse is working, but if the fuse blows and the circuit involved high enough voltage, you'd have the same concerns that keep the V input and COM terminal clearance separated as they already did.
Only final NRTL testing can tell for sure. Hioki could use a special PCB chemistry or conformal caot to provide high insulation.
In the DT4281 PCB, a similar issue is present. One can see the traces on the PCB for each input jack.
Compare to the traces of NRTL marked meters, the Keysight 1271a and the 87V, you can see how wide the distances are between terminals and better yet, they are cut out for the blast shield to improve isolation.
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