Surge protection compared: with or without an external backup fuse?

Surge protective devices (SPDs) fulfil the same protective function. However, the protection concepts differ in their structure and, in particular, in how they behave in the event of a fault.

Conventional SPDs require an external backup fuse, which makes planning harder, means more space is required and increases complexity. With DEHN's ACI (Advanced Circuit Interruption) technology, overcurrent protection is integrated into the SPD itself.

The differences become apparent when compared side by side.

How you benefit:

Less effort

No protection gaps

Increased operational safety

Ueberspannungsschutz im Vergleich_1500x1500px

Two protection concepts – two different results

There are two ways to provide surge protection. At first glance, both approaches meet the same basic requirements. But in practice, there is one crucial difference: the response in the event of a fault determines how reliably systems are protected.

Why conventional approaches hit their limits

SPDs with a backup fuse work, but they have inherent drawbacks:

Protection gaps in the event of small fault currents
Delayed disconnection
Additional voltage drop
Complex to plan

They can provide protection, but they are difficult to install and prone to errors.

DEHN ACI: integrated surge protection instead of additional components

The complexity and inherent vulnerabilities of external backup fuses are now a thing of the past. DEHN's ACI technology integrates overcurrent protection into the SPD itself, eliminating the additional voltage drop and making planning and installation simpler. It also means selective coordination is no longer necessary.

DEHN ACI reduces complexity while enhancing safety.

DEHN ACI vs. SPD with backup fuse – a direct comparison

The differences between the two concepts become apparent when they are compared side by side.

Criterion	SPD with a backup fuse	SPD with DEHN ACI
Protection concept	External backup fuse necessary	Integrated overcurrent protection
Protection gap	May occur	Will not occur
Voltage protection level	Increased due to backup fuse	Constantly low
Planning complexity	high (selectivity and dimensioning)	Low, considerably easier
Space requirements	Additional space for a backup fuse	No additional components necessary
Installation	Complex, more wiring	Easy
Operational safety	Depends on design	High, no time delay, no leakage currents
Leakage currents	May occur due to ageing	Will not occur (galvanic)
Disconnection time	Delayed, because mechanical	Immediately
Ideally suited, if …	Specific constraints exist	Safety and simplicity is the focus
Limits	More effort, protection gaps may occur	Limited short-circuit withstand capability

When ACI? When conventional?

The choice depends on your application:

Requirement	Recommendation
Maximum safety without any protection gaps	ACI
Easy, low-maintenance concept	ACI
Limited space in the distribution board	ACI
Minimum planning and installation time	ACI
Structural restrictions	Conventional SPD (only applies to DEHNventil ACI)
Existing backup fuse present	Conventional SPD – Type 2 arresters with ACI technology can be selected where there are increased TOV requirements or where zero leakage current is required

The situation
Solutions that use backup fuses take up space.

The solution
DEHN ACI frees up space in the distribution board.

Your benefits
Can be replaced during operation to ensure maximum system availability.  

ACI technology eliminates the protection gap and significantly increases the system's availability. Conventional concepts still have their place – the key is choosing the right one for the application.

Bernd Moosburger

Red/Line Product Manager

Find the right solution now 

Find the right products for your application and learn more about surge protection without an external backup fuse.

DEHNventil ACI (Type 1 combined arrester) 

Lightning current and surge protection without a backup fuse. 
For the most demanding requirements in lightning current protection. Integrated protection without additional components.  

DEHNguard ACI (Type 2) 

Compact surge protection for the DIN rail.  
For applications where the focus is on simple installation and reduced complexity. 

DEHNguard M DC ACI 

The specialist for high-power charging and fast-charging posts. 
Already fit for IEC 61643-41.

Surge protection without a backup fuse

More interesting facts about DEHN ACI technology and backup fuses in general.