DC LV Battery Power Supply for Motorhome

Various situations could arise, with different conclusions for "useless or useful". 

The fundamental point about the RCD is that it can only operate if there's an external current path between its two sides, other than the conductors that go through the RCD. (Let's call the sides the source and load side; cables could be on either side and are another source of faults from active conductors to external objects if getting damaged, trapped etc.)  Some current has to go through the RCD's active conductors then return by the external path such as earth or a protective conductor, in order to trip it. 


If everything on the source side is truly isolated from everything else, like a floating generator winding with negligible capacitive coupling, the RCD can never operate whatever happens downstream; just the same is true the opposite way round, if the load side is truly isolated. Taking a single-phase case, one of the two active and initially floating conductors at the load side of the RCD could accidentally come into contact with an earthed metal object: then the other active conductor becomes live, and the RCD does no good at all for an accidental contact between this one (also on the load side) and earth, as the whole fault loop is in the load side. Ok - this requires two faults together, but the first could happen long before the second and be unnoticed.


If instead several RCDs were used for different outgoing circuits, they'd at least protect against cases where faults happen on different circuits. If there is a fault to earth in the source or in possible cables between the source and RCD, and a further fault to earth on the other conductor on the load side of the RCD, then it also can operate.  So if you have an isolated source that "might go wrong" an RCD isn't quite useless.


An old guide HSE 482/2 suggests in para-12 using just an isolated system for a small setup (little chance of an accidental unnoticed fault).  It also confirms (para-39) my suspicion that cables are a likely culprit for the faults. Others e.g. who manufacture 'IMDs' (e.g. here) might suggest an isolated system with IMD; it's a neat way to avoid the shock currents that an RCD needs in order to start operating (and has other advantages too, except for cost).  I'm tempted to continue to other considerations of centre-tap and/or resistive earthing (and their pro and con), bonding of exposed parts, purpose of the earth rods that some like to use with every source, etc., but probably the above is enough to reconcile the views on usefulness of the RCD in this thread's case?


[And the above concerns a system with enough voltage to have need of an RCD. If the motorhome really doesn't have anything at 'dangerous' voltage, e.g. inverter-fed loads, the whole RCD thing seems a waste.]

Various situations could arise, with different conclusions for "useless or useful". 

The fundamental point about the RCD is that it can only operate if there's an external current path between its two sides, other than the conductors that go through the RCD. (Let's call the sides the source and load side; cables could be on either side and are another source of faults from active conductors to external objects if getting damaged, trapped etc.)  Some current has to go through the RCD's active conductors then return by the external path such as earth or a protective conductor, in order to trip it. 


If everything on the source side is truly isolated from everything else, like a floating generator winding with negligible capacitive coupling, the RCD can never operate whatever happens downstream; just the same is true the opposite way round, if the load side is truly isolated. Taking a single-phase case, one of the two active and initially floating conductors at the load side of the RCD could accidentally come into contact with an earthed metal object: then the other active conductor becomes live, and the RCD does no good at all for an accidental contact between this one (also on the load side) and earth, as the whole fault loop is in the load side. Ok - this requires two faults together, but the first could happen long before the second and be unnoticed.


If instead several RCDs were used for different outgoing circuits, they'd at least protect against cases where faults happen on different circuits. If there is a fault to earth in the source or in possible cables between the source and RCD, and a further fault to earth on the other conductor on the load side of the RCD, then it also can operate.  So if you have an isolated source that "might go wrong" an RCD isn't quite useless.


An old guide HSE 482/2 suggests in para-12 using just an isolated system for a small setup (little chance of an accidental unnoticed fault).  It also confirms (para-39) my suspicion that cables are a likely culprit for the faults. Others e.g. who manufacture 'IMDs' (e.g. here) might suggest an isolated system with IMD; it's a neat way to avoid the shock currents that an RCD needs in order to start operating (and has other advantages too, except for cost).  I'm tempted to continue to other considerations of centre-tap and/or resistive earthing (and their pro and con), bonding of exposed parts, purpose of the earth rods that some like to use with every source, etc., but probably the above is enough to reconcile the views on usefulness of the RCD in this thread's case?


[And the above concerns a system with enough voltage to have need of an RCD. If the motorhome really doesn't have anything at 'dangerous' voltage, e.g. inverter-fed loads, the whole RCD thing seems a waste.]