Electricity + Control June 2017

DRIVES, MOTORS + SWITCHGEAR ELECTRICAL PROTECTION + SAF TY

MCB – Miniature Circuit Breaker PV – Photovoltaic PVC – Polyvinyl Chloride SPD – Surge Protection Device

Abbreviations/Acronyms

(or derating); Switching functions (switch, switch-disconnector, Make-first-break-last); Mounting method and special environmental requirements.

Current rating For an MCB to be effective it must not trip (must hold) during normal operation (load dependent) and must trip in time to protect cabling during abnormal operation (cable dependent). These two conditions can be expressed as I n(MCB) ≥ I n(Load) and I (CB Max Trip) < I n(Cable) . A common misconception is that a 100 A circuit breaker will not trip at 99,99 A and will trip at 100,01 A. Any over-current device will have actual trip-points different from its nominal rating. An MCB certified to SANS-60947 [2] may have a non-tripping current (I U ) of 105% its rating (I n ) and a tripping current of 135%. Start with the load to satisfy I n(MCB) ≥ I n(Load) . If the circuit breaker is to supply a known singular load, the load’s current rating can be taken directly. In this case, the circuit breaker could be chosen to provide protection against overload of the load equipment too. If the circuit breaker is to supply a combination of loads, the circuit breaker should be sized to supply the anticipated cumulative load. This can become complicated for many intermittent loads or unknown loads e.g. general plug sockets outlets. Sometimes the allowed load is limited to a reasonable value, simply for economic reasons. In this case, it is improbable that the circuit breaker will provide effec- tive protection of the individual loads and therefore each load must incorporate its own protection. Also consider inrush effects. Many loads have an increased inrush current that can last from milliseconds to minutes. A direct-on-line motor can draw a start-up current of 5 to 10 times its rated current when first energised. Instead of overrating the circuit breaker (and subsequently the cable) by the same amount, utilise a circuit breaker with a time-delay curve that exceeds the inrushmagnitude and period of the load. This might require selecting a cable with a short time overload rating in excess of the inrush (e.g. some PVC cables can temporarily exceed the usual 70°C, up to 90°C or 105°C), otherwise the useful life of the insulation will be impacted. With the circuit breaker curve at hand and deratings (if any) ap- plied, select a cable with appropriate cross sectional area, given the insulation and installation method, to carry the worst case trip current and satisfy I CB Max Trip < I nCable . For building installations, SANS-10142-1 [3] provides guidelines for calculating the conductor capacity. This includes installation method, ambient temperature, number of cables in proximity and insulation. Using a slightly larger cable than neededmakes for a conservative design. For very long cable runs, it might also be required to increase the cable size to limit the voltage drop under full load. If the protected circuit contains branches, each of these cables must individually cor- relate to the circuit breaker rating, regardless of whether their normal load will only be a fraction of the total.

Voltage rating It goes without saying that the circuit breaker voltage rating (U e ) is determined by the source voltage. Be sure to select a breaker to the maximum work- ing voltage of the system. It is often overlooked that a lead- acid battery with nominal 12 V level can be charged over 14 V, or that the open-circuit voltage of PV solar panel array may be significantly higher than its nominal usage.

Correctly specify an ac or dc breaker – these devices are generally not inter- changeable. With dc the direction of cur- rent flow is of importance too. Keep in mind that in battery applications you would need a reverse feedable circuit breaker, as the battery can both charge and discharge. The over-voltage category is selected according to the position in the installation. Surge energy is highest near the service entrance and will dissipate towards lower levels of distribution. For usual 240 V distribution, preferred withstand voltage (U IMP ) is 6 kV at the service entrance, 4 kV in distribution and 2,5 kV at the load, but will also rely on coordination with suitable Surge Protection Devices (SPDs) throughout. Short circuit rating The source also determines the short-circuit rating (I CU ) re- quired, as the prospective short-circuit current depends on the source voltage and source impedance. In a low voltage mains distribution board, short-circuit currents of several kiloamperes is possible. Instal- and I CS

• A circuit breaker is a critical safety device to protect people and property where electricity is distributed. • Its primary function is to protect the distribution system from reaching damaging temperatures. • These over-current devices protect us, our homes and industries.

take note

June ‘17 Electricity+Control

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