Author Topic: Brymen BM235 10A fuse silently destroyed by a switched off UPS leakage current..  (Read 6957 times)

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Offline hggTopic starter

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Hi,

I have a Raspberry Pi plugged in a UPS which I switch off when not used.
Recently I've noticed that the power on led of the Raspberry blinks and fades
away every 5 seconds with the UPS switched off.

I measured ground to live and ground to neutral on the UPS output socket
and they show 229V!..   

Is this normal?
Should I throw the UPS in the bin?
When I switch on the UPS the connected devices work normally.



I've tried to measure any leakage current to ground and the BM235 showed nothing, instead it
started showing 'DIODE' when I connect the leads..   I took off the 10A fuse and it shows no continuity!
Like its damaged. I doubt that more than 10A passed from live to ground on a switched off UPS.
The fuse looks new, no explosion were heard, no sparks were seen.   :)

How is this possible ?   :-//






Thanks.
 

Offline Cnoob

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You UPS may have your neutral isolated from your earth unlike true mains mine reads 247VAC between Live and Earth and 1.8VAC between Neutral and Earth which is a ghost voltage (induced Voltage).
As for anything wrong with your UPS I don't much about them but it doesn't surprise me you are reading mains level voltages between your earth live and neutral.

But your RPI is connected to your UPS by a USB connection and that's where your fault could be. leaky  capacitors seem to cause problems a lot more now.

As for your Meter, They don't have a manual on TME and my BM257 manual doesn't say anything about blown fuse indication, but I am going to take the fuse out of it and connect it inline in a circuit.
« Last Edit: March 05, 2019, 02:58:20 pm by Cnoob »
 

Offline Cnoob

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I took the out the 11amp HRC fuse the meters display read zero amps or as good as damm it.
So I took the 400milli Amp HRC set the meter on milli amp range and plugged lead in Milli/micro circuit back on
still read zero current flowing.

It could be a feature of your meter or something is wrong.
 

Offline hggTopic starter

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The Raspberry Pi 3 is connected to the mains output of the UPS with its own power adaptor. 
Its not connected via USB.

The fuse does not show any continuity.  Its toast.
I am at a loss how this fuse got damaged by connecting it from live to ground of the UPS !!
 

Offline Cnoob

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First off then your power adaptor is at fault.
Second if you connected Live to ground/earth with your meter set and connected to measure current  (Amps) then you shorted it and that would cause the fuse to blow. But If you were measuring voltages then it would not be the cause of your 11 amp fuse to blow.

It isn't unknow that fuse just seem to go open circuit without a specific cause.
 

Offline hggTopic starter

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I've connected live to ground while the UPS was plugged in on mains, but switched off.

This is supposed a quality fuse that can stand at least 10 Amps for some time.
The house RCD would have tripped for just a few milliamps.
 

Offline stj

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the rcd wont trip, the output is isolated from it.
the output may be spiking because of the battery monitoring circuit.
if you had a scope you could see if it's very short pulses.
 

Offline IanB

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A UPS is an "uninterruptible power supply". In other words it is not designed to be switched off when something is connected to it, because that would interrupt the power to device. I think you might be using it in a manner not consistent with its design.

Secondly, it might be that the UPS has some kind of capacitor in the output stage, and if this discharged through a short circuit (a meter in the amps range is a short circuit) then it could easily blow a fuse.

I would recommend you unplug devices from the UPS when it is not in use.
 

Offline hggTopic starter

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This is a very simple and cheap UPS.  Are you sure that the output is isolated from mains?

"I would recommend you unplug devices from the UPS when it is not in use."
That is the way that you use your devices in a blackout though...

Maybe an output capacitor blew the fuse.  This is not your usual fuse though.
There was absolutely no sparks or noise.  Strange.

When I connected only the first lead of the multimeter to the UPS live,
it started doing a thing like autoranging, bust fast.
« Last Edit: March 05, 2019, 03:39:15 pm by hgg »
 

Offline IanB

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"I would recommend you unplug devices from the UPS when it is not in use."
That is the way that you use your devices in a blackout though...

I don't think you understand. In a blackout, the UPS is supposed to provide full mains power on the output from its internal battery. In other words, devices are meant to be plugged into it when it is switched on, not when it is switched off. What would be the point of having it, otherwise?

Quote
Maybe an output capacitor blew the fuse.

It's pretty certain that full mains power from the output would blow the fuse...

(Under no circumstances should you ever connect a DMM to a power output socket while the DMM is set to measure current.)
 

Offline hggTopic starter

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"In other words, devices are meant to be plugged into it when it is switched on, not when it is switched off"

I understand.  The UPS has a switch off though and I have not read anywhere that you should not switch it off
with the devices plugged in.

"(Under no circumstances should you ever connect a DMM to a power output socket while the DMM is set to measure current.)"

Maybe you did not understand.
I connected the DMM between live and ground to see if there was any leakage current.
Is there a capacitor connected between live and ground inside the UPS?

 

Offline Cnoob

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Quote
Is there a capacitor connected between live and ground inside the UPS?

Without seeing the circuit we don't know.
As you said it's cheap, one cost cutting exercise is the use of autotransformers.

And you found out the hard way not to use a meter too short out the Live to Earth.
 

Offline hggTopic starter

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I guess you are both right.
I've shorted live to earth...  :palm:
 

Offline hggTopic starter

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I did not expect to be any current though with the UPS switched off.

That fuse is a beast.  I had a very hard time breaking it open with pliers!
Maybe that is why it was so quite.  The only thing I can see is some
black grains.



Is there any remote possibility that this would be genuine ?
https://www.ebay.com/itm/Bussmann-DMM-B-11A-1000V-DMM-11A-Multimeter-Fuse-for-Fluke-87-V-88-V-287-289-179/261536107575

« Last Edit: March 05, 2019, 04:35:21 pm by hgg »
 

Offline Cnoob

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I suspect those ebay fuses are too cheap But I will leave it up to you.

If you want to measure earth leakage put the meter inline with earth wire or use a clamp meter.
Rule of thumb (but not always)  measure current on the low side/return.
« Last Edit: March 05, 2019, 05:00:10 pm by Cnoob »
 

Offline hggTopic starter

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Indeed.
I measure leakage with a clamp meter.

In this case the UPS was off, it could not power on any device, apart from the blinking
led of the Raspberry barely, and I was curious to see why there was 230V between
ground and live and ground and neutral but with no power to turn on even the Raspberry.

But in order for the fuse to blow there should have been a capacitor between live and
ground inside the UPS.  Is this even possible?  Wouldn't that short the output during
normal use?

I still don't understand how the fuse blew...
 

Offline Cnoob

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If your earth is connected to part of the UPS circuitry, then there is your potential current path.
 

Offline malagas_on_fire

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A extension mains socket or single socket with included switch either for testing or turning off all loads when UPS is completly  powered off is also suitable.
If one can make knowledge flow than it will go from negative to positve , for real
 

Offline hggTopic starter

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UPS OFF
Ground - Live : 230V
Ground - Neutral : 230V
Live - Neutral : 0V

I've connected an incandescent bulb between live and ground and the house RCD tripped.
The UPS does not isolate mains when switched off but it cannot also power anything from
live to neutral.  I have two devices connected.  A 3D printer power supply which is earthed
and the Raspberry Pi with its power adapter which is not earthed.  The Raspberry power
ON LED switches on and fades off every 5 seconds, only when one of its USB ports is connected
to the grounded 3D printer.  So it looks like its a kind of grounding issue.

I think I will include a main power switch as well.
 

Offline malagas_on_fire

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Quote
I've connected an incandescent bulb between live and ground and the house RCD tripped.

This is a dangerous move :S You've conected a resistive element between live and ground, so current will flow and RCD trips , probably more current and arcing :S The RCD has a self-test button to check this feature. Use the NCV /  EF ( Non Contact Voltage) to check isolation, but some cheap UPS have no isolation , even in the battery terminals .

About earth check if not creating ground loops  due to bad adaptor for the RPI, or use a 5V power supply that has grounding so the same loads has the same ground from the UPS. 
If one can make knowledge flow than it will go from negative to positve , for real
 

Offline hggTopic starter

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Its a crappy UPS.
I will use a mains switch for now and I will buy a better UPS with proper isolation.

What is the worst case scenario if the RCD had not tripped?
 

Offline IanB

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Its a crappy UPS.
I will use a mains switch for now and I will buy a better UPS with proper isolation.

I'm not sure why you think a UPS is meant to provide isolation. A UPS is not an isolating device, it is a backup power device. A UPS is meant to be installed and operating 24 hours a day to protect critical equipment that is not meant to be have its power interrupted.

You are using a the UPS in a manner not intended. I do not see why a more expensive UPS will be any different. A good UPS should not even have an on/off switch, since that leaves open the possibility of accidentally interrupting the power to the protected devices.
 

Offline malagas_on_fire

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The idea of mains switch is to use between the UPS and the Loads you want to disconnect, instead of turning off the UPS. Besides the battery if SLAB or AGM needs maintenace charge to avoid wear over the time
If one can make knowledge flow than it will go from negative to positve , for real
 

Offline Johnny B Good

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UPS OFF
Ground - Live : 230V
Ground - Neutral : 230V
Live - Neutral : 0V

I've connected an incandescent bulb between live and ground and the house RCD tripped.
The UPS does not isolate mains when switched off but it cannot also power anything from
live to neutral.  I have two devices connected.  A 3D printer power supply which is earthed
and the Raspberry Pi with its power adapter which is not earthed.  The Raspberry power
ON LED switches on and fades off every 5 seconds, only when one of its USB ports is connected
to the grounded 3D printer.  So it looks like its a kind of grounding issue.

I think I will include a main power switch as well.

 That sounds suspiciously like a Y cap issue with the wallwart providing 'half live' leakage current that slowly charges a smoothing cap on the RPi's 5 or 3.3 v rail until there's enough to trigger an on board voltage regulator (buck switching type?) into life to provide a pulse to drive the LED which then discharges the smoothing cap and the cycle repeats over and over (the classic relaxation oscillator effect).

 With most (likely all) line interactive UPSes, only the Live output is switched from the incoming mains across to the inverter output. The earth and neutral remain connected with the neutral side of the inverter output bonded to the incoming neutral (which is crossbonded in the mains supply wiring to the earth either at the consumer unit or even as far back as the sub-station (or pole transformer or whatever).

 There's normally a 'disable switch' on a UPS so that when the protected load(s) are switched off, you can still keep the battery pack float charged without risking a pointless discharge of the battery in the event of an outage during times when backup power isn't being called for (and also when you want to move the UPS from one location to another or even put it into temporary storage or transport it across town or county from one location to another - longer term storage requires removal/disconnection of the battery pack).

 Bearing the fact of the single pole changeover switching in the UPS, it's extremely important that you don't reverse the Live and Neutral connection to the mains supply. The better Line Interactive UPSes actually incorporate mains polarity connection fault detection LEDs to warn of such problems. If you have the Live/neutral reversed, this could readily account for the winking LED via the Y cap in the RPis's smpsu wallwart.
John
 
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Offline viperidae

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Could be the ups is only switching the neutral line. That would explain the voltages.
 
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