supplementary consumer unit in TT/IT system

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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A second N-E bond is not required, and better not done. I'm not really seeing a reason to add one. If you do have one, it makes the earthing system ineffective at addressing a loss of neutral fault, as no fault is detected (by the installation or by the end users) until there is no neutral feed, at which point lots of exposed metalwork is live, yet the appliances all look dead. With separate E&N lines, this doesnt happen.
You will of course want an RCBO somewhere to provide a circuit that meets 17th edn requirements.
What is nonstandard about the install? Other than it being local generation & earth rod with no RCD or ELCB, which I'm not convinced is compliant even with the 14th edition. In fact wasnt that outlawed in the 1950s with EEBAD installs?
NT

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On 19/05/2012 10:17, MAT wrote:

However, ready access to earth inside the buildings (in both figurative and literal senses) by the sounds of it?

Thus rendering the install TT to all intents when running from the genset...

VFX being a model/brand name for a combined battery charger / inverter with changeover switch I take it?

So you are in effect TT from here on even when running from batteries via the VFX.

That seems a bit odd. At this point I would expect to see a RCD of some sort - even if a fairly high trip threshold type with a time delay.
(unless the earth spike is very substantial and long, its unlikely to offer a low enough impedance on its own to act as an effective enough earth)
This would typically be in the place of the main switch of your "switchbox" (which sounds rather like a 2 way CU?)

Where is this isolator? Prior to the main CU, or in the submain that feeds the outbuilding CU? or are you talking about the main switch inside the main CU?

Fair enough, although the same comment applies about top level RCD for equipment / fire protection. (the new CU can have its own RCD for shock protection)

For the avoidance of doubt, a diagram would help here...

I can't see any reason why you want want to...
If we are treating this as a 16th edition style TT installation with cascaded RCDs or multiple RCDs, then in the event of an earth fault, we only need a small fault current (i.e. in excess of the RCD trip threshold) to isolate the relevant part of the installation.
It makes more sense to me to only have one bond between N & E at the origin of the installation, since that is the common place that the various IT supplies (Main Genset, Solar PV via the inverter & supplementary genset) feed in. (in fact one could argue that if the main genset is a distance away, then it would be better off without its own earthed neutral connection at that end).

Unless the main earth connection is that good (i.e. sub 1 ohm), then talk of "substantial earth wire" seems rather moot, since it sounds like you are not going to be able to pass a significant earth fault current due to the high external earth impedance. Even if it is that good, then it would still seem simpler to design the whole system such that it does not depend on this to operate safely - i.e. like a traditional TT system.
--
Cheers,

John.

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Yes - this is an "Outback" FX model. Works in two modes: as an inverter, or depending on user intervention, or system conditions, as a charger (2KW!) and pass-through from the generator.

MAT > > From one of the MCBs there is a Consumer Unit. All but one outputs are

JR > That seems a bit odd. At this point I would expect to see a RCD of some

MAT: apparently it is - in excess of 6 feet long, according to somebody who saw it go in.

I've specifically checked on this today. The output from the inverter goes directly to a Steeple Main Switch Incomer (insulated) which is fitted with a DSB63 63A DP Main Switch and a couple of MCBs. There is NOT a RCD in this box - just the pair of 32 Amp 6kA Type B MCBs one of which feeds a CU as detailed previously - only ONE circuit from that has a RCD - and that feeds another shed - which has its own earth too.
MAT>> There is a master double pole isolator

JM > Where is this isolator? [...] are you talking about the main switch

Yes - the main DP switch on the DIN rail for all the MCBs (and the one RCD) inside the CU.
For what it's worth this was signed off by a firm specialising in outdoor, multiple source, non-grid installations who get a lot of MOD and FC contracts.
It's starting to look a bit wrong to me - but I'll need to be very sure of my facts to call them on it ...
thanks for your comments,
MAT
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On 21/05/2012 22:48, MAT wrote:

Even 6' into clay would not guarantee a low enough earth impedance
(I find 4' into similar soil conditions round here will give anything from 7 - 20 ohms)
A 32A socket circuit protected by a type B MCB will need a *total* earth loop impedance of 1.15 ohms at the furthest point from the CU.
Also it sounds like any of these sockets could easily be used to power an appliance outside (and for that matter, depending on what the floor "inside" is made out of, even that might count as "outside" for electrical purposes.

Ah, 63A sounds like an odd capacity for a main switch - but quite common for a RCD...
Have a look at:
http://content.yudu.com/Library/A1guxl/SteepleConsumerUnits/resources/3.htm
Page 3, bottom right suggests a DSB63 is actually a RCD with a 30mA trip threshold, and not just a main switch.

Have a close look at that main switch just in case...

ok, so supply from genset / solar PV inverter goes first to the two way unit, thence via one of those MCBs to the "main" CU, and then one way from that CU feeds a another CU. Is that from a single device in the main CU (i.e. an RCBO) or from a separate RCD/MCB pair on a split section of the main CU?

Which makes that RCD sound more like a RCBO...

Indeed - its not easy to give reliable advice without being able to see it myself, but from what you describe so far, I would be surprised if there were not an RCD somewhere need the head end of all this lot...
--
Cheers,

John.

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On 21/05/2012 22:48, MAT wrote:

What documentation has been provided (if any)? There really should be some, but it might be stashed in the filing cabinet of somebody who doesn't understand a word of it. Have you enquired?
Questions in my mind are:
The big genny (how big?) is earthed locally. Is the line to the cabin 2-core (Line + CNE) or 3-core (L, N, E). The armour of a 2-core SWA cable is valid as a protective conductor, provided it's properly glanded at both ends. If there is a separate earth core, what happens to it at the cabin end?
Does the VFX thing provide any isolation, or are its input and output neutrals solidly bolted together? Are the manufacturer's instructions available?
Points:
Nothing here needs to rely on a TT or IT system. All sources are local and under your control, so TN earthing can be employed throughout.
The VFX (source) and cabin mains distribution appear to be wired as a PNB system - a variation on TNC-S. The source neutral is earthed in the first (2-way) isolator box, by dint of the strapped busbars and connection of the main earth electrode. Thereafter it should be conventional TN-S (SNE) to the two consumer units (so no N-E strapping in those!).
Despite TN earthing, RCDs (as RCCBs and/or RCBOs) should be used. The inverter in particular may have limited output s/c current, to protect the semiconductor devices, so you can't assume that the normal disconnection times will be met, based on its output impedance under normal load. And then there's all the usual 17th ed. reasons for RCDs too, as well as the general nature of the place and its users.
--
Andy

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