Selecting Circuit Breaker Size for Surge Protection Device

As always, you size the circuit breaker to protect the wiring, so whatever is the maximum(steady-state) current that the wiring will safely carry will inform the choice of CB rating. Down here in the South Pacific, I would likely wire with 2.5mm2 conductors and use a 20A CB, but your local rules are likely to require something different to this.

SPDs typically use MOVs (that's Metal-Oxide Varistor for the uninitiated) these have a high resistance until a voltage above the breakdown voltage is applied at which point the resistance drops to a low value. It is known that MOVs occasionally fail short-circuit if they conduct a sufficiently-energetic surge, so the CB is used as protection should this occur.

You don't say what you want it for. Domestic is simple, buy a combined SPD+MCB unit, industrial speak to the manufacturers.

It would be interesting to see you order a breaker to handle a fault let though current of maybe >10KA

For commercial building purposes TonyS.

The next question in mind is would a surge (say from lightning strike) with a magnitude 10~100kA which will last some microseconds would be able to let through any breaker without breaking it (I guess it will)

Whats the best time response for a breaker, in your experience/knowledge?

I started to get the idea, that in the event of a catastrophic surge, SPD's fail first then next the breaker for the L-PEN fault event which cause by Varistors in short circuit.

Therefore, it would be realistic (perhaps, if no one object) that size of breaker will be based on the Ampacity of the conductor used, i guess. Though, smaller size breaker greater than earth leakage current rating would also be working fine but not recommendable.

Pls. correct me if I am wrong, i am driven by intuition down here.

Any inputs is welcome.

Intuition? You really need to be driven by your local electrical standards, Mildred!

To know the science of how they come up on this local electrical standards Murphy is a little bit more interesting.

Though irrelevant!

Sorry this is all UK based.

The only ones I've had dealings with for LV were combined SPD-MCB designed to be fitted in consumer units* (CU). They included a 20A (I think) MCB. I'm not going looking at the CU in my flat, its 02:00 here.
They weren't cheap, about £70 for a single phase unit. Connection was by 4mm² tri-rated single cores direct to the out going side of the CU's main incoming isolator and the main earth. The three pole units were a bit bulky as they took up 6 slots in the CU, the single pole only 2 slots.
I used either Hager or Schneider depending on the make of the CU. Most makes will fit most CU's but I'd rather everything be of the same manufacturer. The Hager units were rated for 10KA.

* panel, fuse board, distribution board depending where you are.

Alright thanks TonyS.

Why do you need to know the MCB rating? Most manufacturers give the ratings in their literature.

The units I referred to above are rated for most LV applications, both domestic and commercial. For LV industrial systems I'd start looking at the overall design of the system.

Thanks for the asterisk!

To me, a consumer unit was a quantity (of anything) small enough to be sold to the end user, as opposed to a wholesale unit.

SPD is not a lightning arrester. These are 2 different subjects entirely. Surge protection is for the voltage going into a house or business. If a facility has a large load started in the morning, it can and will drop the voltage on the utility supply. It can also cause a spike in the utility supply when disconnected. That is what SPD is for. It also does not require circuit protection.

Hi,

SPD manufacturers will give a recommendation on the nominal rating of the MCCB - but to me, whether 60A or 100A is totally inconsequential considering that lightning surge currents are in the order of 100KA. As mentioned in an earlier post, the end-of-life characteristic of MOVs is short-circuit. Therefore, the circuit breaker should provide short circuit protection for the SPD circuit should this happen. KA rating should commensurate with the prospective fault current at the point of installing the SPD. Use double pole of single phase SPD and 3-pole for 3-phase full mode protection SPD.

Regards