[SOLVED] Why two Fuses (in Line and Neutral) blows simultaneously?

[muthukumar_ece2004]

In our project, the mains circuit have two fuses in series with Line and Neutral. The Fuse rating is 1.6A and our product only consume maximum current of 0.6A@250V. We have assembled 5 products. While testing one of unit fails and we troubleshoot the problem by opening the unit. And identified both the Fuses are blown.

Why both the fuses are blown at the same time? and what are may be the reason to blow the Fuse like current surge? But i don't thing that the current surge might blown both the fuses. The fuses are slow-blow.

 

[tpetar]

I dont think that is smart to use fuse on neutral. What will happen when only that fuse blow.

You didnt mention kind of project, and some information about that, did you use motors, large capacitors, some higher power bulb as halogen,....

 

[FvM]

I dont think that is smart to use fuse on neutral.

Some fused instrument power connectors are using fuses for both lines. You should consider, that these instruments are usually suppied by unpolarized wall-outlets, in other words, there's no defined line or neutral. I think, the idea behind is that you get a fast disconnection of ground faults indpendend of the abitrary connection of line and neutral.

For the same reason some instrument design use internal double fuses. Personally I never used it.

Why both the fuses are blown at the same time?

This can be surely expected with sufficient overcurrent, e.g. due a shorted transformer, X-capacitor or input rectifier. It's not worth to discuss it in detail. Find the short and fix it instead.

 

[muthukumar_ece2004]

This can be surely expected with sufficient overcurrent, e.g. due a shorted transformer, X-capacitor or input rectifier. It's not worth to discuss it in detail. Find the short and fix it instead.

Our mains circuit consists of two fuses followed by a X-Capacitor (between Line & Neutral) and Common Mode Choke. None of components shorted or fault. We are analyzing in the sense of energy storage in Capacitor and Choke. During first power-up, there is no functional issue in working of unit. Then unit held for more than 12-hours and tried to test again. In the second attempt, unit failed instantly when AC power-on. This is may of due to energy storage in the component during first power-on.

Today, we tested the unit by replacing new mains circuit board. Again failed in the second attempt but this time only Phase Fuse is blown. Still we could not able to conclude on this. May the problem arises due to high energy storage in the choke? and Why this time Neutral Fuse is not Blown?

 

[betwixt]

I doubt it's energy stored in the choke, that would be lost within mS of the power being removed. It's just possible it's residual charge on the X2 capacitor but unlikely, in any case you should have a resistor of say 470K across it to discharge it for safety reasons. Without it, the pins of the power plug could have voltage left across them.

If sometimes only one fuse blows, it points to a high surge current being the problem and one blowing is cutting the current before the other has chance to follow it's demise. Please tell us what circuitry follows the filter, it sounds like a power supply needing a surge to start up. Possibly a thermistor in series with the AC to limit inrush is needed.

Brian.

 

[muthukumar_ece2004]

I doubt it's energy stored in the choke, that would be lost within mS of the power being removed. It's just possible it's residual charge on the X2 capacitor

yes, i mean during powered-off, the residual current in the choke flow in reverse direction and so the X2 Cap charged. But How it will damage the Fuse during next power-up either it will create peak spike or so?

I have a doubt here, X2 Cap have enough charge after powered-off due to choke's reverse current flow. Then after sometime, is there any chance to discharge back through choke to load during powered-off?

 

[FvM]

Your explanations referring to an "energy storage" problem can be hardly correlated to electrical theory, I think. As betwixt mentioned, a non-discharged X-capacitor can increase inrush currents. I doubt that it will blow a 1.6 A slow-blow fuse however You may want to mention the X capacitor value. I would rather exoect other causes of inrush current, as also mentioned by betwixt.

 

[muthukumar_ece2004]

a non-discharged X-capacitor can increase inrush currents. I doubt that it will blow a 1.6 A slow-blow fuse however You may want to mention the X capacitor value.

The X2 capacitor value is 100nF. Could you please tell me how much energy this Cap can able to store? and will it enough to blow 1.6A fuse?

 

[dustmote]

I would think that it would be highly unlikely for that small of a charged capacitor to cause such a high inrush current as to blow a fuse... slow blow or not.

 

[FvM]

100 nF causes a reactive current of 7 mA with 230 VAC.

The stored energy of 100 nF capacitor is about two orders of magnitude below the fusing energy of a 1.6A slow-blow fuse.

 

[muthukumar_ece2004]

AC Line Filter Board schematic is attached here. The X2 Cap value is 0.1uF and Fuses are rated to 1.6A, Choke value is 27mH/1.4A.

Could you please explain the schematic on what happen while switch on, switch-off and again switch-on?

 

[FvM]

Do you mean that the fuse is blown without a load connected at the filter output?