Slate roof, cellar inverter and an isolator the customer wants hidden
A 6 kWp array of 15 x 400 W modules (Voc 41.2 V, Vmp 34.5 V, Isc 12.4 A, Imp 11.6 A) is to be fitted to a 1900 slate roof on a three-storey Victorian house. Two roof lights sit in the proposed field, and there is a lead-lined valley and a clay chimney stack. The inverter is a Class II transformerless unit with an integrated DC isolator, to be mounted in the cellar next to the consumer unit, 28 m of cable route from the array. The installation is TN-C-S, there is no lightning protection, and the customer has asked for the external DC isolator to be mounted 'out of sight at the back of the loft so it doesn't spoil the front of the house'.
1. Assess the mounting approach for this roof and the design decisions that follow from the roof lights, valley and stack.
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Fixing: the slates must not be drilled. The correct method is roof hooks or brackets fixed to the battens/rafters, with the slate above each hook dressed back and re-laid, or a manufacturer's slate replacement flashing. On a 1900 roof the battens may be soft, so the condition needs checking before the design is fixed. Steps in the method statement: careful slate removal, storage and refitting by someone competent with slate, and acceptance that a small number of slates will break - budget for replacements matched to the existing slate.
Layout judgement:
- Set the array out around the roof lights, the valley and the stack with the correct maintenance and fire-access gaps, and keep the array out of the valley's water path.
- Shading from the stack is the real risk. Model or measure it rather than guess; if the chimney shades the array mid-morning or afternoon, either split the array into shorter strings on separate MPPTs, use optimisers or move the array. A single 15-module string with one shaded module under a bypass diode loses far more than one module's worth.
- Confirm no module sits where snow will slide off the valley or where foot traffic for chimney maintenance will occur.
Cable route: the DC cable should not be laid in the valley or across the roof light, and must be supported and mechanically protected where it leaves the array.
2. Respond to the customer's request about the DC isolator location, and explain what BS 7671 and good practice require for the DC switch-disconnector.
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The request should be declined, politely but clearly, and the reason explained in writing.
Requirements that drive the location:
- The DC switch-disconnector must be capable of being operated by a competent person without hazard, be clearly identifiable, and be lockable so the array can be isolated for maintenance or by the fire service.
- It must be readily accessible - a loft hatch behind stored boxes, or a position only reachable by crawling, fails that test and will be failed at commissioning or by a subsequent inspection.
- A switch-disconnector that is not adjacent to the inverter also needs to be labelled and, where the array is remote from the inverter, the guidance is to provide isolation at the array end as well as at the inverter.
- Section 712 of BS 7671 also requires warning notices at the origin of the installation, at the meter position and at the DC isolator, stating that the installation has a dual supply and the array must be isolated before working.
Practical answer: the external DC isolator goes in a lockable IP65 enclosure at a sensible height on the gable end or in the meter cupboard - not the front elevation if that is the concern. The larger point to make to the customer is safety: a firefighter or an electrician needs to find and operate that switch quickly, and 'out of sight' is not an acceptable design outcome. Record the objection and the advice given.
3. Decide whether the module frames must be bonded to the main earthing terminal on this installation, and justify the answer.
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On the facts given, supplementary/protective bonding of the array frame is not required.
Reasoning:
- The inverter is Class II (double insulated), so the DC side is separated from the AC installation and there is no path by which a DC fault can put a dangerous potential on the frames. The frames are therefore not exposed-conductive-parts of the AC installation.
- There is no lightning protection system on the building, so there is no requirement to bond metalwork into an LPS or to control arcing between the array and the LPS.
- The module frames are factory-bonded internally to the module's own protective arrangement (where applicable) and the mounting rails are earthed through the inverter's own earthing terminals only if the manufacturer says so.
What changes the answer:
- The inverter or mounting manufacturer explicitly requires the frame to be earthed - then follow the instruction.
- A lightning protection system is present or added later, in which case the array and its metalwork must be assessed against the LPS and bonded where required, with conductor sizing taken from the lightning protection standard rather than from BS 7671 alone.
- A Class I inverter with an earthed DC side, or a DC-side earth fault detection arrangement that requires a reference earth.
Where bonding is provided, use a conductor sized to Section 712 requirements (minimum 6 mm²), run it in a way that cannot be disconnected accidentally, and record the decision - including the reason bonding was not installed - in the handover pack, because the next inspector will ask.