While surge protection devices (SPDs)  are designed to withstand sizeable single surge events, having a backup fuse can provide stronger protection for electrical assets. 

The Australian Lightning Protection Standard AS 1768 and the Wiring Rules AS/NZS 3000 maintain that an SPD should be protected by the appropriate Over Current Protective Device (OCPD) such as a fuse or circuit breaker, following the manufacturer’s recommendations. But why is it necessary to have a backup fuse for your SPD? The answer can go beyond normal circuit protection.

How Surge Protection Devices Work

SPDs are used to safeguard electronic devices and appliances from voltage spikes or surges. They work by diverting excess voltage away from the power devices, preventing damage caused by power surges.

SPDs are generally installed at the main electrical panel to protect the entire electrical system or within specific equipment to protect individual devices. By safeguarding electrical assets, the SPD can effectively extend the lifespan of electronic equipment.

Having a backup for SPDs is crucial to ensuring an uninterrupted power supply during electrical surges and becomes an added layer of security against equipment damage.

Why Backup Protection is Necessary for Surge Protection Devices

Over time, an SPD becomes susceptible to device failure. An SPD generally experiences two types of failure:

• A gradual end-of-life process due to the natural degradation of the internal component, commonly a varistor; or
• A rapid end-of-life process due to a catastrophic event beyond the scope of the SPD’s rated performance

Backup protection can help mitigate these types of failures. The essential requirements for backup protection include:

• Not reacting in response to a surge or impulse
• Operating quickly and reliably in the case of a short circuit fault of the SPD
• Providing backup protection against overheating and thermal runaway of the SPD

The Importance of Selecting a Suitable OCPD

As with any installation, the OCPD for an SPD must be suitable for the prospective short-circuit current from the point of application. 

Most manufacturers nominate the maximum size of backup protection for surge protective devices. This ensures that the SPD can operate with surge capacities up to its rating, whilst being protected against damage from excess currents. 

The gradual deterioration of the SPD can be safeguarded by the ability of the backup protection to limit the thermal rise in the temperature of the unit. In the case of a varistor, it is necessary to isolate a degraded varistor before it reaches the point of thermal runaway and becomes a fire hazard. The degradation process can be attributed to the following factors:

• Aging of the MOV – This can be due to extreme, repeated stress of high current surges, particularly those related to lightning-induced impulses. The aging process is usually very slow; however, it is dependent upon the level and frequency of impulses that the installation has been subjected to.   
• Sustained temporary over voltages (TOV) - This can affect the conduction of the MOV, causing the SPD to conduct low-level currents for extended periods, resulting in a progressive rise in temperature.  

These incidents can lead to an explosive destruction of the SPD. Under these conditions, the thermal protection capability if the backup protection will react too slowly to prevent the catastrophe. As such, the OCPD must be capable of short-circuit protection.

Given the characteristics needed for an effective OCPD, let’s explore how a fuse or circuit breaker fulfils the requirements.

Are Fuses or Circuit Breakers Suitable for Your Surge Protection Device?

One of the criteria noted previously is that the OCPD must not react in response to a surge or impulse.

Traditional circuit breakers and fuses are designed to protect against ‘normal’ overloads and short circuits based on the standard supply network. When dimensioning a backup fuse or circuit breaker for an SPD, it is important to consider the behaviour of these protective devices when presented with an impulse current from lightning or other surge events.  Normally, the speed of the surge event is too fast for the traditional time current curve of the circuit breaker or fuse, but this may need further investigation. 

How Fuses and Circuit Breakers Can Withstand Strong Currents

There is a difference in the way a fuse or circuit breaker disconnects a short circuit current, compared to the way it will disconnect an impulse current —particularly lightning impulse currents of 10/350 waveform. This occurs due to the frequency and amplitude of the impulse current. 

A fuse can enter its pre-arcing stage, while the magnetic protection of a circuit breaker can begin its tripping reaction at low levels.

Tests on NH fuses, when subjected to 10/350μs impulse currents, show that a 35 A DIN00 fuse can start to melt at around a 4 to 15 kA surge of impulse current.

Circuit breakers of a similar current rating will also enter their pre-tripping function. It is for this reason that many surge devices have maximum upstream protection levels of up to 125 A or higher.

Best Practices for Fuse and Circuit Breaker Installation

The Wiring Rules provide the ideal current rating of an HRC fuse or circuit breaker used as backup protection for a 40 kA SPD as 32 A, and a 100 kA SPD as 63 A. This is considered the best practice for domestic applications, such as households.

 In the case of 32 A upstream protection, a 40 kA surge protection could be restricted to a lower kA surge capacity in the presence of a lightning-induced impulse current.  This is caused by the reaction of the upstream circuit breaker or fuse opening the circuit as low as 15 kA. 

In a domestic installation, this level of protection could be adequate, considering factors such as reduced exposure risk, cable impedances, lightning current splitting, and more.  A 40 kA SPD in a domestic application is still considered a good practice, as there are advantages in having a larger kA capacity to increase life expectancy of the SPD. The OCPD must also coordinate with the service protective device of the electricity distributor. 

For larger installations and areas of higher exposure, the sizing of the upstream protection can be more critical, and the manufacturers recommendations should be considered so that the maximum capacity of the SPD can be utilised.

How to Safely Install Backup Protection for Surge Protection Devices

The application of backup protection for SPDs needs to be carefully considered. Safe installation allows the backup device to do its job effectively, however, improper installation could compromise device performance.

Fortunately, there are some simple guidelines to ensure safe application.

• Follow the manufacturer’s instructions regarding the size of the upstream protection.
• Keep in mind that some SPDs have integrated backup fuses designed and tested to provide the necessary protection levels.
• Special SPD disconnectors are available that combine the ability to protect a small power current value and have the capacity to withstand high surges. 

Proper installation and maintenance of backup protection devices contribute to the overall reliability and longevity of the electrical system.

IPD offers application assistance to ensure safe and optimal site protection. We provide surge protection solutions from leading manufacturers like Dehn, that come equipped with integrated backup fuses capable of safeguarding terminal equipment and Novaris, that offer SPD disconnectors. 

For more information, contact IPD at 1300 556 601 or visit our website.