Choices for electrical installations – selectivity or backup

Choosing the right path

The protection system of an electrical installation comprises of a hierarchy of protection devices like circuit breakers or RCDs that should be able to protect an electrical installation by switching off faulty circuits, while maintaining power supply to the healthy sections as far as possible.

These are actually competing goals, because if you want the system to be 100 percent available, this in theory means avoiding all power outages, whether the result of needed maintenance or power failure. Protection devices would thus be set to allow for considerable delay before a circuit is tripped.

On the other side of the ledger is a deep concern to protect the load and system components. Damage from a short circuit fault depends on how much current is involved, and for how long.

Why selectivity

Since huge amounts of current can be released very quickly, good engineering practice leans towards protecting electrical equipment with devices that are sensitive to minimal fault currents, and that react as quickly as possible.

If you choose 100 percent protection, you may have outages to deal with; and if you choose 100 percent system availability, you may have high amounts of equipment damage to deal with.

In fact selectivity can be total, or partial. Total selectivity means selectivity is guaranteed for all short circuit current values up to a maximum value corresponding to the minimum breaking capacity between the two breakers.

Partial selectivity means selectivity is guaranteed up to a certain level of short circuit current. If the short circuit current in the point where the downstream circuit breaker is installed is lower than this value, selectivity is guaranteed; otherwise, it is not possible to ensure that, in the case of a short circuit, only the downstream circuit breaker will trip.

Selectivity is also a matter for protection against electric shock by automatic disconnection with RCD. It is often necessary to coordinate RCDs intended for fault protection and/or fire protection (typ. 300mA) with RCDs for additional protection of socket outlets (30mA). With time delayed Type S-RCDs, it is the RCD directly upstream of the fault that will be tripped in a final circuit, which leads to the highest degree of availability of the power supply.

Why backup?

In the event of a short circuit, circuit breakers open at a certain current value. The greater this current, the greater the size and cost of the breaker. Breaking capacity must therefore be chosen based on the short circuit current at the installation point, which decreases moving from the power-supply source to the loads.

The characteristic of backup is the ability of a breaker located upstream to help the breaker downstream to clear a short circuit, thus virtually increasing its breaking capacity. This feature enables reducing the size of the downstream breaker, and consequently the overall cost of the system, while maintaining an optimum level of safety.

With backup protection, the discriminating power is often sacrificed in favor of the need to “support” the devices downstream that have to break short-circuit currents beyond their breaking capacity. Backup characteristics for breakers are supplied by ABB, which declares that the upstream protection device increases the breaking capacity of the downstream protection. In other words, for example the S200 circuit breaker with 6 kA breaking capacity can be installed to protect a circuit with short circuit current higher than 6 kA, if an S750DR (SMCB) or S800 (MCB) is installed upstream.

Backup is also a matter for RCDs without integrated short-circuit protection (RCCBs) since they have only reduced capability to clear short-circuit currents. Manufacturers of RCCBs have to give clear indications how to protect the RCCB in case of short-circuit – usually by coordinating with an upstream circuit breaker (or fuse).

With SMCBs (selective main circuit breakers) there is a technology available, which combines Selectivity and Backup in a perfect way. For each short-circuit in a final circuit the downstream MCB protecting that circuit will trip – and the SMCB as main protection device upstream will help to clear the fault by additional current limiting without direct tripping. In this case the MCB is backup protected, the total breaking capacity is increased and the combination is selective over the whole range of short-circuit currents given for that combination.

Combine selectivity and backup?

The principles of selectivity and backup, although applied to the same devices, are normally opposite. Selectivity implies that the product downstream will open first in the case of a fault, in other words that the product upstream is less “sensitive.”

To guarantee selectivity, the upstream protection will not trip. Backup implies that the device upstream helps the device downstream to break, increasing its breaking power. With backup, the upstream protection device actively intervenes to protect the line. It is therefore clear that backup and selectivity cannot be combined between two ‘normal’ protection devices, but that each one offers a particular advantage.

Choosing a solution

Selectivity ensures operational continuity in the non-faulty lines if another line should trip out. Backup contains the overall cost of the system by using protection devices with reduced breaking capacity.

A number of technical solutions can be adopted to coordinate protection devices in a power network. What type of coordination to use in a particular zone depends on the network and its design parameters, and relates to the particular compromises that have been made in terms of reliability and availability, balanced against costs and risk containment within acceptable limits.

A system designer must choose a solution for each zone that offers the best technical and financial balance, taking into account acceptable risk levels and availability of the system; reference value of the electrical quantities; the costs (protection devices, control systems, interlocking components, etc.; the effects; allowable duration and outage costs; and future evolution of the system.

For each of the proposed solutions, there is a combination of ABB products available that will meet each of these needs.

To learn more about selectivity and backup and find the best coordination choices for ABB devices please check out our SOC – selectivity optimized coordination tables and our Selectivity mini-site