Selection of Circuit Breakers or Fuses for SPDs

With more residents in India having interruptions of their electrical supply due to sudden power cuts, unexpected voltage spikes and extreme weather events (like lightning storms), more and more homeowners are looking at surge protection devices (SPDs). However, it is even more often left unsaid that SPDs need protection as well, typically in the form of a Miniature Circuit Breaker (MCB) or fuse.

In straight forward terms, if your SPD travels down the route of failure or an internal short, the only thing stopping the SPD from overheating or catching fire is the breaker or fuse you have installed with the SPD! It is for these reasons that choosing the appropriate circuit breaker and/or fuse for an SPD is not a simple matter, as it affects the safety of your home.

Why an SPD Needs to Have a Breaker or Fuse

An SPD works by diverting excess voltage (for example, from a lightning strike or grid surge) to earth. In this process, the SPD continues to absorb voltage until it is no longer able to exceed its aggerated rating. In that case, the SPD can become a risk for short circuiting. The OCPD (Overcurrent Protection Device), your MCB or fuse, will automatically disconnect the SPD if/when it shorts out and thus keep your system safe.

If you do not? You are leaving the main panel exposed to catastrophic faults and fire hazards.

Key Considerations for selecting an MCB or Fuse for use with SPDs

1. What type of SPD is it?

• Type 1: (typically found at service entry points & generally recommended, especially in lightning prone areas) will typically require high-breaking capacity fuses, often between 125–250 amps, for example, gL/gG fuses.
• Type 2: (typically found in main, or sub-distribution boards) will typically work better with C-curve MCBs or D-curve MCBs, depending on the SPD specifications, with most fuses being rated at 20 amps to 63 amps.
• Type 3: (point of use) could be built into a power strip but in these instances there is generally an upstream fuse or MCB located in the sub-panel.

2. Where does the manufacturer's data sheet specify the correct MCB / Fuse rating?

Manufacturers (either whole units or components), such as Legrand, DEHN, OBO Bettermann specify what fuse or MCB rating and tripping curve is correct. Using the incorrect ratings will certainly void the manufacturer's warranty or the SPD failing certification tests.

MCB or Fuse? Which One’s Better?

Real Life example

Let us say that you have installed a Type 2 SPD rated with a discharge current of (Imax) 20kA. If you have then selected a normal (and ordinary) C16 MCB (which trips too quickly), the SPD will invariably be disconnected, well before a normal surge, the SPD acts. In this instance, the current board with a C30 or C40 MCB may allow it enough tolerance, before tripping, to allow for the SPD. This is essentially the crux of the argument.

Likewise with Type 1 SPDs, more so with homes with rooftops solar and inverter back-up, a robust fuse will required in a premise to withstand the massive energy potentially generated during a lightning surge.

Smart Installation Considerations

• Keep the leads (< 50 cm) between the SPD and the breaker/fuse as short as possible.
• The SPD must take its own MCB, not share with any other load.
• Match the breaking capacities of the breaker or fuse to your local fault levels. If you're in India, use MCBs rated to 10kA in lightning-prone areas.
• In the case of SPDs for solar panels, use DC-rated fuses or breakers. The thermal inertia of AC rated devices is not designed for the arcing produced by DC currents.

Conclusion

Choosing the right SPD is step one. Protecting the SPD with a breaker or fuse rated for the normal fault levels completes a surge protection system for your home. Without this coordination, you could have:

• Induced failure of the SPD
• Nuisance tripping
• Electrical fires

Consult the SPD datasheet, know your load, know your risk, and coordinate the devices accordingly.

The safest way to ensure everything is linked to your yield and maintain your safety is to have a licensed electrician audit what you have in your set-up, or you could always get in touch with us! (You can book a Free Electrical Safety audit too!)