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Topic Title: Insulation Resistance Solar DC Circuits
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Created On: 07 November 2017 12:59 PM
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 07 November 2017 12:59 PM
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Hello all,

Im lookiong for advice on the minimum insulation resitance value on Solar DC circuits (strings/arrays of modules).

BS7671 2008 Section 612.3 table 61 shows circuits >500V tested at 1000V DC should be >1.0MOhm.

IET COP for Grid connected Solar PV Systems section 5.11.2 table 5.4 shows systems >400kW, minimum resitance is 3KOhm.

So which one is the answer?
 07 November 2017 02:13 PM
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Sounds like you're comparing apples with pears there - a 400kW system you'd be looking at over 1,600 panels (at 250W/panel) - whereas BS 7671 figures normally apply to one circuit or one DB's worth of final circuits (or, traditionally, up to 50 points).

- Andy.
 07 November 2017 02:14 PM
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I would say that the wiring, tested alone, should have an insulation resistance of at least 1 megohm, in line with usual practice.

The installation including the PV modules may reasonably have a much lower insulation resistance to earth.
A PV module has a large surface area, over which otherwise inconsequential leakage currents add up.
 07 November 2017 02:16 PM
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The two tables are for different things.

BS 7671 Table 61 is aimed at distribution and final cricuits, switchgear and accessories, with all electronic devices disconnected where appropriate.

Table 5.4 in the IET CoP is for the insulation fault threshold for the insulation fault monitor (typically in the inverter), and relates not only the DC cabling, but the inverter array as well.

Section 16.3 of the IET CoP describes the tests on the DC side of the inverter, and recommends that BS EN 62446 is used to do the tests, NOT BS 7671. This is further described in Section 16.1 of the IET CoP.

EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd


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