Hi!
In most of the official schematics this is stated: "The primary Case ground connection from a inverter
charger like a Multi or a Quattro, must be connected to the Central Negative Busbar of the DC system. Size of this cable must be identical to connected DC negative".
However, the connection to the ground makes it impossible to have a cable that size.
What is the reason against simply connecting the negative and the ground together before the busbar to save on cabling?
The case must be grounded for safety. Otherwise a fault might make it live at full ac voltage.
Hi, picking up this post again.
I'm thinking about the same thing It's not physical possible to do this.
We have the 48V version so the problem is not as bad, I can get on 25 mm2 on maybe 35 mm2. But I use 50mm2 on the DC side and that I don't think will work.
If we where to go on the same cable dimension as on the AC side it would not be a problem.
Anyone have a opinion?
In the AC world, the safety ground connection provides a path for fault current to avoid a chassis, etc from reaching line voltage. If a fault occurs, the safety ground connection needs to carry enough current to trip the circuit breaker. This happens rapidly so the wire won't heat if there is some voltage drop. As an example the National Electrical Code requires only a 6 AWG wire for a 200 amp service even though the ampacity of #6 is only about 50 amps.
Conditions are different in a DC system. The currents are much higher and resistance of the safety ground could be high enough to prevent the fuse/circuit breaker from tripping. Voltage drop of 20' of #6 wire carrying 1000 amps is 10 volts and would not blow a 1000 amp fuse!!! You can do the math to see what's needed in your system.
