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Brymen BM235 10A fuse silently destroyed by a switched off UPS leakage current..
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Electro Detective:
Low Z mode is great for stray leakage and confirming capacitive ghost voltages, but will trip the RCD every time, unless DUT is isolated from the mains.
FWIW: some meter brands need to cool down (up to one hour) after using Low Z mode on mains voltages,
as resistance readings in Ohms mode may not be stable or accurate with the user believing he/she has a lemon meter   :-\
 

I've played with UPS units in the past and noticed on some that the input Live and Neutral can be wrongly wired by the manufacturer,
especially "international market wiring colour scheme" snafus > black and white vs brown and blue vs red and black
vs drag revue pink and purple   :palm:

AND I've observed Live and Neutral flipped/reversed on the UPS output during a power outage as referenced to the input sine wave 

Some can be wired properly so that input and output POLARITY is correct, with some expected voltage/current phase shift difference expected

By saying 'Some can wired be properly' I mean if the rewire does not affect the battery charging circuit and battery if there are common connections.

If the UPS output runs through an isolation transformer, like some APC SmartUPS and similar Compaq/HP IIRC, those are easy to correct -if- they are out of wack.
Yes, some are, most not, luck of the draw.
Still, their transformer output is isolated (many times via two 120v legs plus CT feeding 240v to L and N output) with earth/ground still connected at the wall socket,
so the connected load/equipment won't care



All my blab above assumes correctly wired wall socket/s, IEC kettle cord leads, decent power strip boards,
with no moisture or short causing dirt/metal particle ingress anywhere.
and non flipped or poorly repaired DIY extension cords (classic widow makers)   :scared:


@ hgg: we may never know if the meter fuse blew because of what happened somehow with the UPS, or from a previous test, a severe meter drop/vibration, 
or the fuse was a rare dud,
or someone borrowed the meter and zapped it with voltage on the current setting, whatever... 

The good news is that it got you pumped to find out what the go was, and corrected a far bigger problem/issue
...than worrying about the euros you will blow on another fuse     :'(
   
YES, they are WAY-Y-Y overpriced for what they are,
another necessary credit card rape item
but still a bit cheaper than buying a replacement meter and or blowing money at a hospital/funeral

...and the sellers know it   8)


The sneaky cheaper knockoff fuse floggers aren't doing the electronics community any favours either  **** them    :--
hgg:
If I read the fuse datasheet correctly, it can hold 125 Amps for 40ms.
The 30mA RCD can hold for the same amount of time 150mA (500% of its rated trip current)

The 40W incandescent lightbult had an initial cold resistance of 97Ω, which means that
it drew 2.37Amps when it was connected between live and ground.

When the fuse was connected from live to ground it was a dead short (almost)
How can we calculate the amperage at the time of the short?
The device was connected to a 10A fuse which did not blow.

(I am trying to understand because there was not the tiniest spark when I connected the
multimeter probes and one probe was barely hanging by itself on the ground terminal.)
Cnoob:

--- Quote ---(I am trying to understand because there was not the tiniest spark when I connected the
multimeter probes and one probe was barely hanging by itself on the ground terminal.)
--- End quote ---

That could suggest the fuse was already broken.
And why the RCD didn't trip it could be that when you shorted it out there was already another current path to complete the circuit.

You tripped the RCD with a light bulb, you may have blown a expensive fuse by shorting mains level voltages and you wired up your UPS
incorrectly.

To me that suggests you should leave mains and mains level voltages alone.
Jeroen3:
If I switch off my UPS, it goes into bypass mode. Maybe only interrupt neutral indicates a problem inside the UPS?
If I want to power down anything behind my UPS, I have to switch if off, and unplug it.
dmills:
RCDs have TWO parameters, a rated trip current, often 30mA (actually this is a 'must trip' level, they generally go at half of this), AND a trip time....
Common examples have a must trip time of 30ms, and will generally go in 20ms.

These two parameters mean that there is ample time for a stiff mains supply to eat a 10A fuse  and clear the fault before the RCD trips. Yea, sure, 1kA or so of PSC (Could be much higher if close to the incomer and using heavy cables) is way more then 30mA, but if the fuse opens and quenches the arc in under 10ms the RCD will probably not trip because the fault has not been sustained for long enough. 

You can actually buy time delay RCDs which are usefully deployed upstream of the usual sort as they provide substantial fire risk reduction while still providing the discrimination to avoid tripping the whole installation due to some stupid downstream fault that should just trip the local one. Standard sizes for these are (100mA, 100ms), (300mA, 300ms) and the biggie for large three phase jobs, (3A, 3 seconds).

Note that a 30ms delay 100mA RCD does not provide useful discrimination with a 30mA, 30ms RCD because  it is a crap shoot as to which one will trip first on a earth fault of more then 100mA.

The fuse did its job, and THIS is why meters have expensive fuses on the high current ranges. 

Regards, Dan.
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