I’m planning to use NH fuses in a disconnect holder to enable me to isolate the power at several points in my new off-grid system. I have 2.5kW PV generation per string feeding a Victron 250/60 MPPT controller to charge a 48V battery (2x 24V 200Ah Victron Lithium Smart connected in series). I have a Phoenix 48V 5000VA inverter to connect to the batteries. At some point in the future I want to be able to expand the system by adding further identical PV strings, MPPT chargers, batteries and inverters in synchronised mode.
Given the PV DC output peaks at 230V and 13.8A I expect to need a 20A fuse for each string. I want to use an NH blade fuse to handle the high voltage and enable me to use a disconnect holder as an isolator.
The solar charge controller peak output I’ve calculated is 58A at a nominal 51.2V for the 2 batteries in series. In practice it will be more like 49A at the MPP of the panels. I think a 70A Mega fuse and holder will be fine for this connection to the busbar.
The batteries are rated at 200A continuous output and the inverter will draw 104A at 5000VA max continuous output at 48V nominal input. Realistically that’s more likely to be 78A at 4kW with 51.6V input, ignoring the inverter efficiency and cable losses. Cables will be short (1m) and thick (70mm2). I want to use NH fuses for this part too due to the current - one for the battery cable to the busbar, and one for the inverter cable from the busbar. I assume a 125A fuse will be good for both these.
I found the Jean Muller Keto battery disconnector here
My question is: since the NH 00 fuses don’t seem to be available with only 20A rating, can I use the smaller NH 000 fuses in the NH 00 holders? I want the isolation functionality of the disconnector fuse holder so I don’t need extra DC switches.
Thanks in advance for any advice or comments on my calculations.
Fuses in pv-primary circuits are not really useful, as operating and short-circuit currents are very close to each other, in some cases the operating current is even higher then short-circuit.
For PV usually this style fuse is used to fuse each string, along with a DC PV breaker to dieconnect an array (because you should never open fuses under load unless it is an emergency)
The NH fuse disconnectors are designed for disconnection under load for industrial applications and have a much higher breaking capacity than enclosed isolators. They are effectively a knife switch with big contacts and far less likely to suffer damaging arcing when disconnected under load. They are used for currents of hundreds of amps at 500 or 690V AC and many are rated at 250V DC or more. That’s why I want to use them - very little to go wrong.
I’ve re-read the specs of the NH disconnectors and it looks like the NH000 fuses are the same length as the NH00 but slightly narrower so they should fit the 00 holder fine. The max rated current of the 000 fuses is 125A and 00 fuses is 160A. I may go up a size for the inverter disconnector to NH1 which are rated up to 350A from what I can see. The voltage is of course far less of an issue for the inverter than for the PV cable.
I’m interested in the comment that the operating current of panels can be greater than the short circuit current. I don’t see how that’s possible. Short circuit current is measured with virtually zero resistance - any other measurement will involve a small but finite circuit or load resistance and therefore the current must be less.
NH disconnectors can be disconnected under load, but you’re supposed to wear full arc-protection equipment when you do so even for AC, so that’s even more necessary for DC where the arc stays longer.
Look up the power draw of the fuses in their data sheets; my 250A NH1 fuses, for example, use 18W at rated current. A switch will probably use way less power.
Personally, I have chosen NH directly at the batteries for their interrupt capacity, but I also have Victron battery disconnect switches in the circuit so the fuses are intended to only protect, not disconnect.
Assuming a fuse is required there is going to be some power dissipation in it regardless of whether I use a fuse link disconnector or a separate DC isolator switch. In the case of a switch, it’s impossible to know if any damage has occurred due to arcing inside the switch body. At least it’s visible on a disconnector.
Anyone have any thoughts about the reliability of isolator switches?
I am just working on a busbar with NH000-fuses. The bar will connect my batteries, my chargers and my inverters.
The fuses will protect my Pylontech US5000 with 100A each.
Tomzn is exactly one of those companies that i would not buy from. CE is only a statement from the company that declares “they solemly belief” everything is conform to european standards. I only belief in a ul-listing.
I agree completely. The whole point of these safety devices is to stop your house catching fire (plus other benefits, but that for me is the main one).
Chinese products are intentionally marked to make people think they’re tested for safety by copying the CE mark but making the space between the two letters slightly different so they can “claim” it’s not really a CE mark if challenged. In fact they have no proof of safety whatsoever and I would automatically consider them highly dangerous. You only have to look at the toys imported from China particularly at Christmas which are sometimes extremely hazardous for children but come with a “CE mark”.
Please, everyone, don’t use Chinese “safety” parts as they’re more likely to set your house on fire than if you didn’t have a safety device fitted at all.
