Quite often, customer will ask, which rating of MCB or RCDs to be chosen to protect the surge protective devices (SPD)?
The answer is simple: we do not know.
The fact is that different MCB has different tripping force, different designed tripping mechanism has different moving contact and static contact and their arc running systems are also different.
Some 20A MCB can pass 8/20μs 20kA, others type 40A MCB may trip at 8/20μs 20kA.
Therefore, we need to test the backup MCB with the surge discharge current before we can answer this question.
There is one rule we can follow: MCB or RCD should not trip at In, but are allowed to trip at Imax. This is supported by IEC61643-12 Clause 18.104.22.168.
When SPD becoming fault with constant high voltage and current, it may burn like a torch. For example, voltage above Uc with 100A current will make the SPD emit flame nearly 0.6 meter long . To choose the fuse as backup protector for SPD is actually not a good idea. For In:20kA Imax:40kA SPD, the recommended fuse is 125A, so even at 100A fault current, the fuse will not break the fault current. The backup fuse in this case does not provide the protection for the SPD and the distribution board where the SPD is installed.
To solve this problem, our company has designed a special breaker to protect SPD which called TY-SCB. The TY-SCB will trip at low current, which is 3 ampere. And the TY-SCB will allow the nominal discharge current (In) pass the breaker without tripping. The SPD can normally withstand 5A current, so before it caught fire, the SCB already cut the circuit. TY-SCB is the ideal protector for SPDs.