Short Circuit Protection at Downstream Converter/Inverter

I really doubt your short circuit calculation and/or your understanding of the power distribution that you have. Without knowing your system or your method in doing these calculations though I cannot tell you if you are correct or where you made a mistake. However, if the numbers you've calculated are correct then you can instead use a fuse to protect your wiring. You can get fuses that trip at less than an ampere of current.

Well then, in general terms, you need a smarter circuit breaker (that's assuming that the insufficient fault current is due to the supply system as opposed to too small conductors).

How big is this system (how many kva/kw, voltage(s)) and what is the short circuit you've calculated.

I'd consider something like this:

• a circuit breaker with shunt or undervoltage trip so the breaker can be tripped by some circuitry (note that molded case circuit breakers are available with shunt or undervoltage trip coils, so if your system is small enough to be protected by a molded case CB, you don't have to consider the costs (and size) of a typical ACB)
• current sensing equipment and relays to detect the short circuit current and then trip the breaker (the current sensing equipment might be CTs for AC circuits, or shunts for DC circuits)

You may need some consulting help.

Dear DANSES,

Maybe I can help you to solve this issue. You can use a comparator which name is LM339, and refer the realted the material which find by the Internet. I hope you can supply the schematic to me best.

Best regards to you.

Rory

An over current protection relay 50/51

Dear all,

Thank you for your replying.

As you know, most of inverter or converter have a feature (build in) to shut down when 1.1 or 1.2 times output full load current from capacity have reached. During the short circuit at downstream, feeder outgoing, this feature will be shutdown the inverter. That give me a headache because of selectivity study can not accept this system. Selectivity said only at the feeder fault shall be isolate. However, when short circuit at downstream around 0.36 kA, it will be secure by limiting inverter 1.2% times output full load. Therefore, that system make all feeder outgoing shutdown.

Main question is how to achieve selectivity?? We have search some fast action breaker or fuse, but still can not solve this problem.

We use inverter 800W (24VDC/230VAC). Regarding to 1.2, its equal output current will be limited 4.3 x 1.2 = 5.5 until 6 A.

Assuming you've done your short circuit / selectivity calculations correctly, it might be a tough problem.

I would look at your loads and consider using multiple inverters / converters and divide the load on them based on criticality (whatever your definition might be). Then (under most fault conditions), only the inverter / converter feeding the shorted circuit should trip, providing you selectivity. (It might be a slightly different selectivity than you're used to, depending on how you divide the loads.)

I'm quite certain that multiple (smaller) inverters / converters will cost more than bigger one sized to handle the entire load, so you have to consider the criticality of your loads and whether the extra cost is justified.

Use the maximum amperes out of the controller and the maximum wattage at surge divided by nominal output voltage for the inverter. Take both answers and multiply by 1.25. This will be a good safe calculation. (a little over maybe but safe) You inverter comment and the 230VDC are confusing.

You are interesting.

The topic is about utilizing an existing 230VDC power supply in a specific area.

This is the situation. The only available power supply in such space area of installation is 230VDC which is intermittently turned on and off [75% continuously cycling ...] On this space area is a need to power a device drawing a 5A load at 10VDC. Actually I had one...and another one...but they all burnt because they are not designed for the intermittent factor of being turned on and off...24 hours all week...

any advice is a relief...

You are not very informative.

Many power supplies work quite well with power being ON only 50% of the time for fifty or sixty times a second. I've also seen many supplies that have been unplugged for weeks work flawlessly the rest of the year. If you burn up your supplies regularly and you cannot describe or understand why, have an electrician or electrical engineer look at your system.