On 20.05.19 12:11, Chris Albertson wrote: > How much DC current are we talking about? Let's say the house is running > an air conditioning load and you are doing laundry and the 'fridge is > cycling and you have a few computers on and every light build is turned on > and you own a 200-gallon saltwater aquarium. Let's call it 20 KW of use > in a really big house. This a very-worst case scenario. > > Your DC is set up for 480 volts going into the inverter. 20KVA / 480 is > is 41 amps.
Even there, the fault current is the short-circuit current of the array. That's not too bad for PV, as it's internally strongly current limited. But upthread we were mostly talking about DC from the battery, i.e. at 48v in most cases. As I've mentioned upthread, you can expect a fault current of 10 times AH rating, i.e. 2000 A for a 200 AH battery bank. That's the strength of the arc we must be able to rupture in the event of a fault. Switching normal loads is back to less than 100 A for less than 5 kW, but still hard on contacts. (The contacts on the starter solenoid for an old Caterpillar D6 are blocks of copper with a mm of silver on the surface, to take the several hundred amps at 24v. IIUC, silver oxide conducts.) > It is not hard to find a good 60 or 80 amp switch. You can > buy one on Amazon.com for $120. Switches that can handle high DC loads are > very common. Here is one examle > amazon.com/Siemens-HNF362-600-volt-... > <https://www.amazon.com/Siemens-HNF362-600-volt-Non-Fused-Switches/dp/B005G953DE/ref=pd_day0_hl_60_1/140-9661733-2175833?_encoding=UTF8&pd_rd_i=B005G953DE&pd_rd_r=a160cb45-7b31-11e9-b1ee-5b598291dad5&pd_rd_w=8QFZM&pd_rd_wg=4YekC&pf_rd_p=ad07871c-e646-4161-82c7-5ed0d4c85b07&pf_rd_r=QQK03FTGP5PQVYKFENBP&psc=1&refRID=QQK03FTGP5PQVYKFENBP> > These DC disconnects are a code requirement and are installed by the > million, literally. All the switches I've come across are stamped with their AC and DC current breaking capacity. That must cover the potential fault current, if there's no HRC fuse, or you're building a fire starter. (I.e. you still need the fuse.) > No competent engineer will design a high power low voltage system because > you can no longer us wire, you'd n=be into solid copper busses. The hundred amps only flows from battery to inverter - and yes, the cables are about half an inch in diameter, much like starter motor cables in a car. I'll be running low voltage DC for LED lighting, but that's at modest current. Erik _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
