What fuse do you need for a Type 1 SPD? Robin Earl, Market Development Manager at DEHN UK, provides some answers and warns about deviating from established norms.
The answer to this question is a simple one, and it can usually be found in the installation guide provided with the product. However, some contractors, for whatever reason, still want to deviate from the published guidance, so any change needs to consider the following.
The Type 1 SPD has three different types of current that can flow through the circuit:
1. The lightning impulse current (Iimp). This is what the SPD diverts to ground during lightning events.
2. The short circuit value in cases of a fault at the location of the SPD. This is called short circuit interrupt (Isccr).
3. For spark gap Type 1 SPDs there’s a third value to consider – the line follow current (Ifi).
As an example, for the top of the range DEHNventil, a combined type 1/2/3 SPD, the lightning current value is 25 kA per pole for a total of 100 kA for all four poles.
The short circuit value and the line follow current are rated as 50 kA each.
Any deviation from the specified fuses, which in the case of the DEHNventil is 250A gL/gG, needs to take account of all those values.
The most requested deviation from the specification is to ask if an MCB is okay in the Type 1 SPD circuit. MCBs typically have a breaking capacity of between 10-15 kA for commercial applications, so this is nowhere near the 25 kA required.
We’ve seen what happens to MCBs after a lightning strike, it is not good. We’re aware that MCBs have different ratings, for example to BS EN 60898 the rating is 10 kA and to BS EN 60947-2 it’s now 15 kA, but still not 25 kA. Using a D curve MCB will not alter anything.
If the MCB is the overcurrent protection device (OCPD) in the surge circuit, then the MCB during the act of being blown apart will remove the SPD from the role of an overvoltage protection device. As the SPD is in a parallel circuit, power is maintained to the installation, so the next surge will not be diverted to earth as the SPD has been removed, and further damage will happen.
MCCB usage
We also get asked about using MCCBs as they can have rupture capacities beyond 25 kA. We cannot forget the short circuit interrupt value as well as the lightning impulse and we find that most MCCBs will fail one or the other during tests.
The MCCB may not be fully destroyed by the tests, but it can trip and isolate the SPD. Again, the installation will remain energised, but protection has been removed.
Other fuse types
Finally, we get asked about any other fuse type apart from the gG/gL, and as for the other OCPDs types listed previously, if we have not tested it and its not on the datasheet then proceed at your own risk.
We understand why these questions are asked, and it’s mainly in retrofit situations where there’s a panel board feeding the SPD via a MCCB or fuse and this is the simplest solution that presents itself. Alternative solutions could be a different way to break out of the board via feed thru terminals replacing the MCCB or using an isolator.
Another solution could be the Type 1 SPDs that have built-in fuses as per the DEHNventCI. Then the installation just needs the connection to the phase bars with or without a dedicated surge circuit isolator.
Get more details on Dehn’s complete surge protection range here
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