IEC 61010 HAZARDOUS LIVE 1

[zappedagain]

I'm working on the safety analysis for CE Marking and this is the first time I've had a system that wasn't using a wall-wart for a power supply. So now I have to deal with the MAINS (note I put the 61010 terms in upper case) inside my enclosure going to the power supply.

My power inlet has a 6.3A fuse so it can handle my typical 1.8A load (5.3A if the supplies are 100% loaded). I have wires with BASIC INSULATION going to the power supply (hot, neutral, ground).

Under NORMAL CONDITIONS the basic insulation is fine. If I take a SINGLE FAULT CONDITION of the insulation breaking then the only protection is the fuse. IEC61010 Figure 1 shows that the fuse needs to blow within about 0.040 seconds, but that doesn't work with my typical load.

Am I missing something here? Luckily all of the MAINS wiring is inside the enclosure, so as long as I tell the user to stay out of the box nothing is ACCESSIBLE. That seems excessive though. Is the BASIC INSULATION failing the wrong SINGLE FAULT CONDITION? In reality it shouldn't be a problem unless someone pinches a wire under a bolt while assembling the unit. But that never happens...

 

[7anoter4]

The wire insulation -basic insulation- will probable withstand the usual abuse -electric and mechanic- but in the load side could be a short-circuit.
The shortcircuit current may reach 100-1000 A but the fuse will limit the current to approx.30 A in 5-10 millisec. [0.005-0.01 sec]
Fast acting blow fuse with a breaking capacity of 63 A will blow-up at 30 A in about 1/4 cycle [5 milliseconds=0.005 sec].see:

http://www.netaworld.org/files/ItemFileA190.pdf

 

[zappedagain]

That make sense, the fact that I was missing was how big my current spike will be if my insulation gets damaged. Worst case it would be the far end of the input wire (24" of #20, so 0.028 ohms maximum). Even at 90VAC that will give a 3.2KA spike (over 500x my 6.3A fuse rating) so the fuse will blow well within the time required for IEC 61010. However...

The SINGLE FAULT current is measured with a 2K path to ground representing the human body model below 100 Hz. With a 2K load I only get 0.12A from 240VAC. The safety limit is 0.0005A, so somebody could still get zapped and the fuse wouldn't blow.

Like I said in the first e-mail, luckily the MAINs are only accessible when the covers are off, and a user won't be taking them off. Maybe I stick a label on that warns about high voltages inside. I'll check that out.

 

[geekEE]

Zapped, I would recommend a label like this one:

http://snipurl.com/5dcgy

Glenn

 

[zappedagain]

I like it! I'll run that one by my marketing department.

Z

 

[7anoter4]

The shortcircuit current depends upon all series impedance from the utility transformer, distribution line and finally you wires.
The voltage on the receptacle outlet can drop up to a half of rated than 1-2 kA could be enough.
2) The fuse will not protect a person against electric shock, indeed. You need a residual current device [RCD].
RCDs are designed to operate within 10 to 20 milliseconds and to disconnect
the electricity supply when they sense harmful leakage, typically 30 milliamps.
For most of cases it is fair enough. See:

http://www.docep.wa.gov.au/WorkSafe/PDF/Guidance_note-process/Guide_electricity.pdf

The California University stated that the safe "let-go" current is 0.009 A for men and 0.006 for women.
The modern GFCI [Ground Fault Circuit Interrupter] may trip at only 0.005 A .See:

http://www.synergytech.net/shedding_light.pdf

 

[zappedagain]

Thanks Z, that is some interesting background. For now I'll keep the users outside of the box; if they need to go inside I'll have to add a GFCI (RCD) at the power inlet.

Z