There is a lot of confusion and interpretation of the different requirements of grid tied systems when it comes to the centralised disconnect device (CDD), fireman’s switch, Anti-islanding, and soon to be Dead Grid Safety Lock (DGSL) OR accessible disconnect switch . Let me try to summarise and explain them below:
Centralised Disconnect Device (CDD) – Is something that comes from the NRS 097-2-1 specification, where all SSEG systems above 30kVA would require a CDD. This device would serve as a repeat of the same safety functionality found in NRS 097-2-1 certified inverters. It was however decided in the last NRS097 working group, that these requirements would be removed, as there was no sensible grounds to keep it there. That being said, there are some Municipalities/Metros like COCT that do require the CDD as long as the NRS097 document has not been officially updated. Keep in mind that Municipalities are in their full right to request over and above the NRS 097-2-1 specifications as it mainly serves as a guideline and not a national standard to comply with.
Fireman’s Switch – In other countries, this switch is a requirement. What it essentially does, is to switch off the DC side of the solar PV installation, to reduce the chances of any power flow from the DC side, so that fire rescue personnel could safely work on the roof or use water when there is a fire. According to my knowledge the Fireman’s Switch with the above mentioned purpose, has been disallowed in the updated SANS 10142-1-2 due to two reasons. 1 – The solar PV will always be “on” if exposed to sufficient sunlight, hence it should still be treated as live, even with the DC system disconnected or switched off. 2 – The additional DC wiring and connection for a fireman’s switch actually increases the chances of faulty/bad connections which could result in a fire. In my view, this term should not be used for the SA solar PV application.
Anti-islanding – Anti-islanding ensures that the grid tied system will switch off and/or prevent any power feedback towards the grid when there is a power outage or maintenance done on the lines. Inverters with NRS097-2-1 certification, have this anti-islanding features/functionality built in the inverter itself. The Anti-islanding will look at parameters like voltage and frequency and disconnect when out of bounds conditions are met. Note: Having anti-islanding alone, does not necessarily mean the installation/grid tied system complies with the NRS 097-2-1 certification as more than just out-of-bounds conditions are checked under the NRS 097-2-1 certification, like power quality and noise levels.
Dear Grid Safety Lock (DGSL) or Accessible Disconnect switch – Is already a requirement for Eskom SSEF connection approvals, but could soon be part of the SANS 10142-1-2 requirements. It does not fulfil the same role as the NRS 097-2-1 or Anti-islanding, but serves as a fail safe in addition to the NRS097 requirements, to make sure that no power will be fed back towards the grid during a power outage. For customers with a dedicated feeder/transformer (like farmers, industrial, commercial), they have the option to install an accessible disconnect switch that will allow the utility to switch off the entire SSEG system (disconnect on the AC side) at their point of supply. The DGSL will be a requirement for shared feeders like residential customers.
Make sure you comply with your municipal requirements.