LV protection system design
When designing a low voltage LV protection system, industrial safety, reliability, and energy efficiency of electrical circuits in general is paramount. LV protection systems are necessary devices to protect and prevent damage to electrical equipment due to fault (overloads, short-circuits, etc.). An efficient LV protection system allows to reduce operational and downtimes falls, making it an indispensable added-value component to the design of LV industrial and commercial electrical systems.
The following article will guide you throughout most aspects of designing a protection system for low voltage circuits, covering final protection device selection, system architecture, and much more. After reading this article you should have a clear guideline to design a full-featured LV protection system whose compliance to international standards guarantees the system long-term stability.Circuit protection considerations: Motors may need protection from overcurrent conditions and short circuits, which may certainly also require protection from surges, for instance from abrasive sand, etc.
Fault levels: Knowledge of the possible fault levels, such as earth faults, short circuits, and overload conditions to be encountered in a given installation, is vital to protective device selection.
Types of protective devices used in LV watt and current meter systems
In the small watt and current meter shown in Fig. 22•24 which is modified to include the required circuits for automatic switching of the motor, a linear potentiometer is used.
Circuit breakers: as usually required in the LV meter circuit of 50V. input and approximately 1 amp. full scale deflection, requiring fuses for protection.Overcurrent Protection Relays: These relays sense an overcurrent condition, i.e., the current exceeds a pre-selected value, and may be provided with additional reclosing features to automatically reclose circuit breakers or fuse cut-out switches after the faulted condition has been cleared.
Sizing Protection Devices
Wholesome consideration must be given to the details of size and construction of all protection devices, because if too great a size is forced into a connection, the protective device may be rendered incapable of accomplishing the desired result. On the other hand, if too small a device is chosen, it may fuse without opening circuit breakers. Aspects to consider:
Current Rating—Protection devices must have a rating above all expected currents never to aperate normally.
Breaking Capacity—This is the maximum fault current which the device may safely interrupt. This should be sufficiently greater than the fault current in the system in order to not destroy the device.
Time-Current Characteristics—This curve of a proper time- current characteristic defines the relationship between fault current and the time taken in tripping the protective device. In building this curve, proportionate allowance and detail must be given appropriate circuit limitations.
Selectivity—In the multilevel pattern, it is extremely necessary to select devices in which only the protection closest to the fault will trip.Proper Coordination of Protection Devices
The protection devices must be properly co-ordinated to ensure effective selectivity.
Selectivity means that in the event of a fault, only the protection device nearest to the fault will operate or trip, without affecting the rest of the system. Proper co-ordination is as follows:
Time-Coordination: That is, the various protection devices must be so chosen that they trip in a time-sequence; those nearest the fault first and the others next in budget if necessary.
Current-Coordination: Again the protection devices must be so co-ordinated that the current-actuating devices are those inducing the highest currents, and the devices next in turn those inducing lesser and lesser.
Grounding and Earthing
A properly designed LV protection system must be suitably grounded and earthed for safety, relying on this system for diverting to earth, safely and without danger of electric shock, any current. The following must also be provided as part of the protection system:
Earthing System: The earthing system must be designed for the provision of a low-resistance path into the earth. This may take the form of earthing rods, earth plates or a mesh system depending on the state of the soil.Bonding: Bond all exposed conductive parts, such as metal enclosures and frames to the earthing system so as to prevent electric shock encapsulating hazard.
Equipotential: Necessarily this will be done by making sure all metal parts of the system, even if it is in various switchboards on several different floors, is at the same potential, thus minimising the risk of differential.
Monitoring and Diagnostics in LV Protection Systems
Ultimately, LV protection systems need monitoring and diagnostics so as to pick up early approaches to faults, thus reducing downtime. Ideal in predictive maintenance terms as a way of enhancing reliability. What to look out for?
Fault Indicators: These are devices that show visually a fault condition and assisting the maintenance team in identifying quickly where the fault is.
Current and Voltage Sensors: There are devices that will; . These constantly test and verify the current in and voltage on the system. Alerting if these stray out of sync through incipient fault.
Remote Monitoring: New technologies afford the convenience of remote monitoring. Nice to know operators have their eyes on the ball on `the other side of the pond’.
Testing and Commissioning of the LV Protection System
A final note is all that’s necessary here. Once the LV protection system is fitted, then a test should be made to ensure all protection is functioning.
Functional Testing: Making tests on the functioning of almost all the protection elements.
Coordination (Selectivity) Testing: Confirming all protection is correctly coordinated and that, therefore, selectivity is achieved.
Earth Fault Testing: Ensuring that the earthing system diverts fault currents to earth.
Simulated Fault Testing: Confirm all react using simulated fault conditions.
Conclusion
That’s basically it as far as designing an efficient LV protection system; not often easy and a deep understanding of the principles of protection and the requirements of the electrical installation is called for on the part of the designer. But with protection devices chosen carefully and protection properly co-ordinated, the system is one that will not only enhance safety but that is more reliable and as a direct result more efficient, and in ongoing monitoring and maintenance it will be found that the life cycle of the system all sung pleased to contain.