On Thu, 19 Feb 2009 13:28:56 -0500, "Paul E. Schoen" wrote:
: :"Ross Herbert" wrote in message :news: snipped-for-privacy@4ax.com... :>
:> I never saw anything which indicated that the 20A CB tripped. In fact I :> think the OP said it didn't iirc. : :From the OP's first post: : : "20A industrial circuit with a GFI, backed by a breaker into a : bus fused at 400A. Breaker tripped, NO GFI trip." : : :> I can recall an instance several years back where a client experienced a :> spontaneous fire on the motherboard of a new IBM desktop PC. The SMPS, :> being :> 350W (I think), did not go into hiccup mode because the fault condition :> was such :> that the SMPS was happy to continue to supply sufficient current to :> maintain the :> fire. By the time the fire was up and away the printed circuit board :> tracks had :> melted and disconnected the fault from the SMPS output and the fire kept :> going :> of its own accord. Because the fault condition was now disconnected from :> the :> SMPS it continued to operate normally and was not subject to an overload. :> No :> mains fuse or any other protective device associated with that PC was :> activated. : :The SMPS in this unit was probably no more than 100 watt, and more likely :about 60 watt, and fused at about 1 or 2 amps, because the load was: : : "We're talking two 30 watt HV modules systems here" : :And only one was used at any time. It really sounds like something on top :of the supply ignited and reached a temperature where the aluminum may have :burned, although the lack of Al oxide is puzzling. Apparently there was a :consequential fire involving other flammable materials that may have caused :the line cord to burn. But it's also puzzling why the conductors in the :conduit were melted on both sides of the outlet: : : "Wiring was conduit from the panel to a Ul approved lab : grade outlet strip built along the wall, wiring for 4 feet on : either side of the outlet in use was burnt/destroyed." : :I'd love to see the pictures... : :Paul : :
Thanks, for the precis Paul. Like yourself, we are all left to make suppositions about the details of the SMPS, the input fuse rating etc, in this matter. It might be helpful if the OP could say what these details were. The 20A CB might have only tripped after the fire was well alight and the insulation on the line cord had melted to allow the active and neutral (or earth) to come into contact.
I also surmised that the SMPS would be in the region of 100W, which, if I am correct in remembering its output voltage was 15V, would mean that it would go into overload protection at somewhere around 7.5A. Since there was only ever one HV unit operating at any time, then a 50-60W SMPS would have been adequate and this would have limited the maximum fault current available.
However, assuming a 100W SMPS, as long as the overload condition stays below
7.5A, a current of, say 6A continuous, is quite capable of producing sufficient heat in components (if there is flamable material around) to cause a fire. As I related, this would be similar to the fire situation I described in a brand new IBM desktop pc some years back. The mains input fuse did not blow, and the SMPS continued operating as normal - and the fire kept going. Had not the office been occupied at the time - we all know that many establishments leave their desktops on 24/7 - this could well have resulted in a more serious fire.In a hypothetical situation, assuming that an overload fault condition (not a dead short) had developed in the HV switcher, and which was insufficient to cause the SMPS to go into "hiccup" mode, had there been a E-T-A ESX10 smart circuit protector rated at say 2.2A (assuming 80% efficiency of the HV switcher) on the output of the SMPS, a fault current of around 2.9A (min) - 4A (max) would have tripped it within seconds of the overload condition occurring, and disconnected the fault condition from the supply. Without current to continue the heating process a fire would not occur. This reduced current is well below the maximum output current from a 15Vdc, 100W SMPS under the same circumstances. And even in hiccup mode the SMPS can deliver short duration high current pulses into a fault (not a dead short) which might be sufficient to maintain heated components in that condition.
We all would love to see the photos, but since there is legal action pending, that isn't possible.