Arc flash and UPS cut off when plugged USB cable in!

[eveares]

Just wondering what would have caused the following.

I moved my NAS box from my kitchen to above my hallway data cabinet, and when I plugged the USB cable in between my NAS and the UPS, it shorted out all the 5v USB ports on my NAS.

Unplug and re-plug the USB cable on the UPS end, same thing. Try it one more time and this time as I plug the USB cable back into my UPS, there is big arc and my UPS suddenly cuts out.

UPS and connected devices came back on shortly afterwards, however not sure what caused this problem. UPS is a APC 1500VA UPS and NAS is powered by a SMPS. Data cabinet is bonded.

NAS-to-UPS link works fine with a another USB cable, and I could not detect any shorts in the original 5M USB cable using my DMM what runs up half a meter of plastic trunking (what is packed with power and data cables) from my data cabinet to above the shelf where the UPS is.

I also connect a USB microphone to my router what is also in my data cabinet using this 5M USB A to USB B cable, and all is fine. Mic powers up, Router detects a USB device a second later.

A intermittent faulty USB cable perhaps?

Regards: Elliott.

 

[omega015]

plastic trunking (what is packed with power and data cables)

By packed, how packed? what electrical cables do you have in this? perhaps there was some capacitive coupling going on?

Alternatively it could just be a faulty usb cable in certain conditions (i.e. temperature, when the flex is bent a certain way etc..)

 

[bernardgreen]

The output of an ELV power supply unit that does not have an Earth connected will float by capacitive coupling to a potential somewhere between Neutral ( close to Ground ) and Live ( 230 volts AC ) Most commonly this induced potential is around the mid point ( 115 volts AC ) . As it is capacitively coupled the current it can supply is at most a couple of milli amps so at worse touching the ELV will give a mild tingle or a feeling of a "buzz". When the ELV supply is connected to something that is at Ground potential or has a path to ground the induced potential will drop to the potential of the circuit to which the ELV supply is connected. The human finger is such a path. Other paths are the signal and ground connections between equipment and it can happen that when the connection is made the current along the leads is, for a few micro-seconds, quite high as the induced potential is discharged. This transient peak can affect electronic modules by "locking up" the protection circuits on the inputs to semi-conductor devices such as micro processors and audio amplifiers.

 

[eveares]

By packed, how packed? what electrical cables do you have in this? perhaps there was some capacitive coupling going on?

3 flex Power cables (70W up to UPS and back down to the cabinet, 15W to upstairs room from the cabionet), 9 Cat 5e/6 cables, 1 6-core alarm cable, and the 5M USB cable.

The output of an ELV power supply unit that does not have an Earth connected will float by capacitive coupling...

That makes sense to why a UPS may cut out momentarily regarding the inital and sudden discharge of capacitive coupled voltage from a ELV source being grounded out to the UPS, however providing it was not a temperamental cable or CC from the parallel power cables, why was it only happening with the original 5M USB A to B cable what was also permanently shorting all the USB ports out on my NAS while it was plugged in?

 

[ban-all-sheds]

3 flex Power cables (70W up to UPS and back down to the cabinet, 15W to upstairs room from the cabionet), 9 Cat 5e/6 cables, 1 6-core alarm cable, and the 5M USB cable.

Oh dear.

 

[eveares]

Oh dear.

Just read this: https://www.tlc-direct.co.uk/Book/6.6.1.htm

While I doubt that CAT5e cables and alarm cables insulation are rated at 230v/400V ; However could a data cabinet, alarm panel, etc.. not also count as an enclosure where their are both Band I and Band II cables present?

And what about Band I and Band II cables that constantly pass by each other under false raised floors typically found in office buildings?

 

[Iggifer]

Just read this: https://www.tlc-direct.co.uk/Book/6.6.1.htm

While I doubt that CAT5e cables and alarm cables insulation are rated at 230v/400V ; However could a data cabinet, alarm panel, etc.. not also count as an enclosure where their are both Band I and Band II cables present?

And what about Band I and Band II cables that constantly pass by each other under false raised floors typically found in office buildings?

It's defined in that TLC page as a 'cable enclosure' not an enclosure, so trunking, conduit, tray or basket etc. There is no way around having 230v and 12/24v in alarm panels, fire alarm panels etc. There is very often plenty of segregation inside the unit, the 230 is often far away from the elv on the pcb, and in some cases, the 230 doesn't even connect to the PCB. It goes directly onto a separate transformer.

Cables under raised floors/above dropped ceilings should be run separately. You can get cable basket with a divider in the middle etc.

Not saying they always are, a lot of the time they're just lashed over the ceiling grid, under the floors. Jobs will often start out nicely, but someone either underestimates how much tray is needed, or someone comes along later and has no way of accessing the whole tray under the floor so just lashes it in with rods.

You could just use a bigger piece of trunking with a smaller length inside, or two lengths next to each other