Greetings!
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ HEALTH WARNING! This is a very non standard insulation, unlikely to be met with in domestic situations, and is NOT explicitly covered in things like Darrell Locke's guides or similar. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++This is one that's got a few heads scratching!. The installation is a forestry one with a large wooden building supported on breezeblock piers and massive oak beams. There is NO concrete base and/or UFER type plate.
The building is off grid, but is served by a large single-phase generator, solar panels which charge a submarine type battery and a VFX inverter which provides a 230V AC supply. In addition there is the facility to plug in a portable generator if power is needed when the big genny can't be run and there is not enough battery power.
The big generator has one side of its output grounded at point of installation. It is located some distance from the timber "super shed".. Soil is clay and quite wet.
At the building the generator is connected into the VFX, along with the battery pos and neg. The 230VAC first goes to an isolator switch box with a couple of (one unused) MCBs. Within this switchbox the Neutral and Earth busbars are connected. There is a ground Earth (spike) connected to the Earth busbar in the isolator switch box.
From one of the MCBs there is a Consumer Unit. All but one outputs are simply MCBs of various ratings, but there is one RCB which feeds yet another exterior building. There is a master double pole isolator switch on the rail that breaks all circuits served by this Unit. From the Consumer Unit onwards wiring is standard 16th Ed.
A solar hot water system is to be installed for showers and washing. This requires 230V AC pumps and a control console.
Because of the variety of users of this installation it is desired to maintain the existing main Consumer Unit with the simple "big red switch" which can be turned OFF at the end of the day, thus killiing all existing sockets/lights/permanently wired appliances.
The 230VAC to the solar hot water system needs to be maintained at all times. [1] The proposal is to use the unused MCB in the main isolator switch to feed another small Consumer Unit (as typically sold for garage use). This would then supply power to the Solar system, and also a few radial sockets, security cams etc.
The advantage is that this will NOT be turned off in normal usage, irrespective of "the big red switch" on the main Consumer Unit.
Because of space and access considerations the second Consumer Unit will have to be about 3 metres away from the main isolator.
Now - at last! - the crunch question. given that there has to be some voltage drop in a fault situation between the Earth connection at the isolator (where Neutral and Earth are bonded) and the Earth connection in the second Consumer Unit, ... should the Earth and Neutral busbars in the second Consumer Unit be cross bonded or not.
I'd be interested in answers, together with the reasoning. I've heard both views from both "academic" MIEE types and practical contractors. There is a third possibility which neither side can substantially shoot down - a substantial earth wire from the second consumer unit to the connection where the outside ground earth comes in - and NOT bonding Earth and Neutral busbars in the second Consumer Unit. But apparently a scenario can be imagined where that is less than ideal.
[1] If you don't then sunny weather will cause the temperature in the tubes to rise massively, denature the glycol, cause the manifold to distort, air locks - all sorts of nasties - been there, seen it and don't want it to happen again!Regards,
MAT
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