When I connect a cable from the ac out of a multiplus 2 to a distribution box, do I also have to place a fuse directly after ac out to protect the cable to the distribution box or are the breakers in the distribution box sufficient?
The breakers to ac in and ac out in the distribution box are 25A and the cable is 4mm2.
yes, you should have a single fuse/breaker on the AC out of the multiplus, i.e.as a master switch, for both protection and isolation.
The Db can then have an assortment of other breaker ratings appropriate for their circuits.
I thought about this a bit more. The issue with the cable protection is not over current (that’s protected by the 25A breaker in the main switch board connected ac out) but short circuit.
As the short circuit protection for the c25 at ac in is about 250A the 250A (C25, 10In) extra short circuit protection on ac out doesnt make much of a difference, as the multiplus 2 5k will maybe add 20A to that.
The only issue I can think of is when the multiplus starts inverting again after the short circuit, bringing voltage to the damaged cable again can cause a dangerous situation.
Normal breakers will clear a short circuit in about 10mS. That is too short a time for the peak current to damage the cable, so that should not be an issue.
Where did you install the residual-current device (RCD)?
It must be installed downstream of the Multi, because an input-side circuit breaker is not included when operating in inverter mode.
I have 2 rcd’s of 30mA on AC out before the end breakers.
They’re after the multi too, aren’t they?
Yes,
The total scheme is:
C25 main brraker → 300mA selective rcd → C25 breaker → cable 4mm2 → isolator switch → multiplus ac in → multiplus ac out → isolator switch → cable 4mm2 → c25 breaker → rcd 30mA → B16 breaker → appliances
The second C25 circuit breaker on the output is unnecessary if you are using the B16s, and the Mp will trip accordingly if the main cable is damaged during inverter operation. Provided the mains impedance downstream of the Mp is sufficiently low, the 16 A rating can be used; however, you must have this verified by means of a loop impedance measurement! If this is not adequate, the protection may need to be downgraded! The worst-case design condition is inverter operation (increased mains impedance!).
In normal operation, there are then two C25 circuit breakers in the branch, both of which will/can trip in an emergency.