SMA STP 50-40 Operating Manual page 114

11 Troubleshooting
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 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.
114 STP50-4x-BE-en-23
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SMA Solar Technology AG
Operating manual