choosing fusible resistors
choosing fusible resistors
(OP)
Hi all,
I was wondering if anyone out there has any experience specifying fusible resistors for use as circuit protection in a 3 phase watt hour meter. I've looked around on the web a good bit and have not found any definitive information for how to properly use these things.
Here are my particulars:
VAC input: 45 - 528
3 phases + Neutral
Peak Primary load: 10W (this is not constant, but rather pulsed during cellular radio transmission. Pulses are 1.154ms out of 4.616ms)
Nominal Primary load: 2.1W
This will be feeding an offline flyback circuit.
I know I need to have one in each phase + neutral, but I'm not sure what size to choose and what timing I would need.
Thanks for your assistance.
Regards,
Larry G
I was wondering if anyone out there has any experience specifying fusible resistors for use as circuit protection in a 3 phase watt hour meter. I've looked around on the web a good bit and have not found any definitive information for how to properly use these things.
Here are my particulars:
VAC input: 45 - 528
3 phases + Neutral
Peak Primary load: 10W (this is not constant, but rather pulsed during cellular radio transmission. Pulses are 1.154ms out of 4.616ms)
Nominal Primary load: 2.1W
This will be feeding an offline flyback circuit.
I know I need to have one in each phase + neutral, but I'm not sure what size to choose and what timing I would need.
Thanks for your assistance.
Regards,
Larry G





RE: choosing fusible resistors
Conventional fuses can be purchased to have a wide range of overload characteristics from extremely fast blow, to very slow blow. They can also be soldered onto a circuit board just like a resistor. Something like this perhaps :
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RE: choosing fusible resistors
If the fused load is really only 2 watts average, then how do you fuse a current in the low mA range?
The other issue is using fuses to protect a 3-phase circuit. When one goes, what happens to the other two phases? Do they go berserk and sequentially blow their fuses before something is damaged? CBs for 3-phase are ganged so they all pop at the same time. Hard to gang fuses...
RE: choosing fusible resistors
This circuitry will be in a 3 phase watt-hour utility meter. Not a lot of power will be consumed during the normal operation of metering. However, during the daily transmission of kWh data back to the server, small bursts of power will be used to support the radio transmission. There are super capacitors on board to help deal with the burst and provide hold-up in the event of power failure.
As for the use of resistors vs. fuses, I really don't have a preference. In single phase applications that I have seen, both fuses and fusible resistors have been used. Of course, I'm guessing here. All the info I've seen in the single phase example says "a fusible resistor is used to limit in-rush current and comply with safety agency requirments to blow open in the event of a catastophic failure in the power supply". In-rush current won't be much as I only have two 33uF caps in series.
Another thing I'm not sure about is the voltage rating of the resistors. As this circuit has the potential to operate at over 500V, that could be a problem with the fusible resistors available. The ones I've seen are only rated at 3W 500V max. So...
Not really sure where to go next..
Thanks,
larryg
RE: choosing fusible resistors
Some X-10 modules (those AC remote control gadgets) use a fine gauge wire (soldered directly into the PCB like a jumper) to provide ultimate protection. But that is for the full several amp load, not just a few mA. In other words, whatever you do, you have to make sure that virtually all (or all) of your failure mechanism will draw enough current to reliably pop the fuse.
I guess lightning protection of your circuitry is a whole other major consideration. Adds a nice layer of complication when you try to predict the failure modes.
I suspect that this little power supply is not a trivial design exercise. (Insert nightmare vision of power meter with the glass cover blackened from the inside...) I hope that you've budgeted accordingly.
Do any of your power company customers have any qualification requirements and test procedures? If so, then they might provide some useful clues about the best way forward.
Or peek inside similar products. Or check the patents (very easy these days).
RE: choosing fusible resistors
RE: choosing fusible resistors
Gunnar Englund
www.gke.org
RE: choosing fusible resistors
I am designing an off-line flbyack supply that must run off of 3 phase power. This supply will be inside a commerical watt-hour utility meter. Therefore, the 3 phase will come directly off of the pole (xfmr). I have seen similar designs of this sort (from meter mfg's) and they typically utilized fusible resistors in all lines (3 ph + N)after the MOVs but before rectification. I have never seen fuses for this application. The values of the resistors are typically in the 10-20 ohm range. In addition, the Neutral is also rectified in the event one of the phases is miswired. My question has been: Why are fusible resistors used instead of fuses in this application and if resistors are used, how do you size them?
As I metioned before the load is very light for the majority of the time (25mA). Only during radio transmission does the load significantly increase and even then, it is on 25% DC and supported by 3F of capacitance.
I think perhaps the reason that resistors are used instead of fuses relates to the operation requirements. Since this is a meter that measures kWh and will indicate how much power to charge for, we probably don't want fuses to pop due to transients or the like and prevent the power company from missing any revenue. Using resistors would allow the meter to continue to function except in the event of a catastrophic failure.
Does this make better sense?
Regards,
LarryG
RE: choosing fusible resistors
Gunnar Englund
www.gke.org