Hardly anything works in industry without energy. A brief power interruption is enough to delete data, disrupt production processes or perhaps even destroy goods in production. According to EN 61000-6-2 and EN 55024, a voltage interruption of just 10 ms, i.e. 0.5 periods, can mean that an AC consumer unit can no longer be operated. Against this background, many companies use uninterruptible power supplies (UPS) in the AC range. The aim is to ensure a constant energy supply even in the event of a power failure. If the power fails on the mains side, these systems are buffered by batteries to ensure operation without disruption or data loss. In many places, however, this is only a theoretical security; in practice, it rarely works. For this reason, E-T-A has developed the 'EBU' circuit breaker as variable overcurrent protection.

Standard protection does not work

With AC UPS systems in common use, the low output power that the devices can provide in the event of a fault is a serious disadvantage. In order to protect its own internal electronics - for example rectifiers and inverters - the maximum current that the UPS can provide is very limited. In the event of a fault, this means that the overcurrent provided by the system is comparatively low, as the UPS immediately minimizes the voltage in the event of an overload. The corresponding limit values can be found in the UPS manufacturer's data sheets in accordance with EN 62040-3.

Typical load limits of UPS systems are 1.5 to 3 times the rated current and are exceeded within a very short time in the event of a short circuit. This results in the UPS shutting down completely to protect itself. This means that the original purpose of the UPS is no longer fulfilled and energy security is no longer guaranteed. In practice, this means that a faulty load on a UPS can paralyze the entire UPS. And this in turn can result in a system shutdown with a wide variety of undefined states.

Circuit breakers as a solution?

For a 10 A circuit breaker with a C characteristic to trip safely, it requires a residual current of 100 A.

© E-T-A

The connected circuit breakers, which were previously intended to protect consumers in UPS systems, are usually miniature circuit breakers. For physical reasons, however, these are usually not capable of tripping in the event of a fault on a UPS: They require a relatively high overcurrent to trip; if they were designed to be more sensitive, they would cause frequent false tripping. However, if the fault current is comparatively low, it takes several minutes in the best-case scenario for the circuit breakers to trip. In many cases, this protective element does not take effect at all. In both situations, the UPS shuts down completely to protect itself and the previously expensive power supply is interrupted.

In concrete terms, this means that if a 10 A circuit breaker with a C characteristic is to trip safely in the event of a fault in an application, it requires a fault current of 100 A in accordance with its characteristic. This is a value that a 10 KVA system cannot provide in most cases - so the fuse ultimately fails in the event of a fault. Consequently, a UPS without appropriate protection is simply not capable of ensuring energy security.

Oversizing as a remedy

The characteristic curve of the EBU can be adapted to the UPS system and load conditions and ensures reliable tripping.

© E-T-A

When users recognize this dilemma, they often solve it in a cost-intensive way: In order to make the thermal-magnetic circuit breakers trip, they buy a high output current by purchasing a significantly oversized system. The idea behind this is that the larger system can provide more power in the event of a fault and cause the protective elements to trip in the event of a fault. However, such a UPS is not only more expensive to purchase, it also incurs higher follow-up costs year after year. Larger UPS systems also require a larger battery capacity. This in turn means higher maintenance costs. In addition, annual power loss costs are significantly higher. Furthermore, the efficiency inevitably decreases if the UPS is dimensioned too high. A high level of efficiency would reduce the UPS system's own consumption and thus the operating costs over the entire service life of the system. All in all, the annual power loss costs alone make such an oversizing unprofitable in this constellation.

In addition, this solution often still does not work if the cable lengths are not sufficiently taken into account. This is because the cable resistance caused by long cables additionally dampens the current that the UPS can supply and thus again prevents tripping in the event of a fault. Even at a distance of 100 m (i.e. a cable length of 200 m), a cable cross-section of 1.5 mm² results in an attenuation of 2.38 Ω at a 230 V voltage source. The 10 A circuit breaker with C characteristic described above can no longer fulfill its protective function in such an application.

This situation often leads to attempts to protect the loads with smaller or more quickly dimensioned miniature circuit-breakers. However, this does not increase safety, but only the number of false trips, for example when switching on switching power supplies or similar.

Stable energy security

An alternative is the 'EBU' circuit breaker. The device for 230 V(AC) consists of a miniature circuit breaker in accordance with EN 60947-2, which is approved for short-circuit disconnections of up to 10 kA. It also has built-in electronics in accordance with EN 60950-1. This intelligence takes over the measuring and evaluation tasks for the protection of a UPS system. The two components of the circuit breaker share this task: if the electronics detect a short circuit or an overload, the device switches off via the circuit breaker.

The Electronic Breaker Unit EBU for 230 V(AC) consists of a circuit breaker and attached electronics.

© E-T-A

The switch is available with rated line protection currents of 6 A, 10 A and 16 A and with B and C characteristics. Users can use it as a 1:1 replacement for the circuit breakers previously used or required by the UPS manufacturer. The 54 mm wide device is operated directly at the outgoing circuit of the respective UPS. With the help of a ten-stage setting switch, system planners can adapt it to the performance of the respective UPS system and the existing load conditions. In the event of a fault, the circuit breaker trips reliably - and only in the affected load path. All other supply lines remain unaffected. This means that the unit can easily tolerate the frequently problematic switch-on processes and the associated higher inrush currents. The EBU circuit breaker only trips if the UPS system reacts with a voltage dip due to an impermissibly high short-circuit current.

The adjustment to the UPS and thus the setting of the selectivity is made by setting the UPS rated current on the switch. The value can be found in the UPS data sheet. For overload protection, the rated load current is adjusted on the unit. Ultimately, the new circuit breaker makes it possible to reduce the size of a UPS system by a third and cut annual energy costs by around 40%.

Author: Tobias Prem is Product Manager in the Industry, Energy & Equipment division at E-T-A Elektrotechnische Apparate in Altdorf.