SMA STP 50-40 - Page 114

11 Troubleshooting
SMA Solar Technology AG
Operating manualSTP50-4x-BE-en-23114
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Figure 19: Schematic diagram of the measurement
Calculating the insulation resistance
The expected total resistance of the PV system or of an individual string can be calculated
using the following formula:
total
The exact insulation resistance of a PV module can be obtained from the module manufacturer
or the datasheet.
For the resistance of a PV module an average value can be assumed: for thin-film PV modules
approximately 40MOhm and for polycrystalline and monocrystalline PV modules
approximately50MOhm per PV module (for further information on calculating the insulation
resistance see the Technical Information "Insulation Resistance (Riso) of Non-Galvanically
Isolated PV Systems" at www.SMA-Solar.com).
Required devices:
☐ Suitable device for safe disconnection and short-circuiting
☐ Measuring device for insulation resistance
Device required for safe disconnection and short-circuiting of the PV array
The insulation resistance can only be measured with a suitable device for safe disconnection
and short-circuiting of the PV array. If no suitable device is available, the insulation
measurement must not be carried out.
Procedure:
1. Calculate the expected insulation resistance per string.
2.
DANGER
Danger to life due to high voltages
• Disconnect the inverter from all voltage sources (see Section9, page91).
3. Install the short circuit device.
4. Connect the measuring device for insulation resistance.
5. Short-circuit the first string.