Understanding Surge Protection For LED Lighting Systems
Surge Protection Overview
Voltage surges have a huge destructive impact upon electronic equipment including LED lighting systems. They wear out LED drivers and distribution panels prematurely, and increase service interruptions to LED lighting. Beyond material damage to the luminaires, voltage surges caused by lightning, for example, can trigger or break protective devices in the circuit boards of lighting distribution panels.
The vulnerability of electronic lighting systems to over-voltages is widely recognized in technical literature, and different international regulations and standards specify the need for lighting protection. This document explains the causes of lightning over-voltages and how they affect lighting installations. It also proposes solutions to maximize protection performance and continuity of service.
Public lighting installations are exposed to the environment. Located where continuity of service is essential, it is crucial that these installations are protected against lightning and over-voltages.
Investing in protection can extend luminaire lifetime, improve public services and greatly reduce overall operating and infrastructure costs.
What are transient or surge over-voltages?
Surge over-voltages are spikes that can reach tens of kilovolts but last for only a few microseconds. Despite their short duration, their high energy content may cause serious problems to electronic equipment connected to the electricity network — from premature aging to destruction — resulting in service disruptions and costly repairs.
Figure 1 - Transient 'surge' over-voltage
Voltage surges have several causes. For example, lightning discharges that directly strike the distribution line of a building, or its lightning rod, can induce electromagnetic fields that generate voltage spikes in nearby lighting installations. And very long outdoor distribution power lines are highly susceptible to the direct effects of lightning strikes, with large currents from the lightning being conducted in the power lines. It’s also common for non-weather phenomena to cause voltage spikes in adjacent lines — for instance, switching devices (contactors) inside electrical cabinets, or the disconnection of transformers, motors and other inductive loads or high power equipment (generators, welders) coupling energy on shared branch circuits connected to sensitive electronic equipment.
Surge over-voltages have two modes of circulation: common and differential as illustrated in Figure 2. Common-mode over-voltages appear between the live conductors and earth: for example, line-to-earth or neutral-to-earth. Differential-mode over-voltages circulate between live conductors: line-to-line or line-to-neutral. A well-protected luminaire should integrate protection for both modes.
Figure 2 - Definition of common and differential-mode currents
Surge overvoltage protection is provided by installing a surge protection device (SPD) on the vulnerable line. In the event of a surge overvoltage, the protective device will divert excess energy to earth, thus limiting the peak voltage to a tolerable level for the electrical equipment connected downstream.
An SPD can be installed in parallel or series.
When the SPD is connected in series it acts like a fuse. So when the priority is to protect the electronic components down the line from further damage, as is the case in most Outdoor applications, series connection is preferred.
When connected in parallel, the luminaire continues to function even after the SPD is damaged. However the electronic components down the line are no longer protected. So when continuity of functioning is preferred over protection of components down the line, parallel connection can be chosen.
The SPD will get damaged after weathering a number of spikes above a certain voltage level. It is vital that the operational indicators provided on the SPD be monitored as part of general maintenance.
Protecting against the effects of surge over-voltages in LED lighting systems
An SPD acts as a voltage-controlled switch. When the network voltage is lower than the activation voltage, the component is passive. On the other hand, when the network voltage exceeds the activation voltage, the SPD diverts the surge energy and prevents it from destroying the equipment. When choosing an SPD, consideration must be made of the equipment’s exposure to lightning, along with the maximum impulse voltage that the equipment needs to withstand.
Figure 3 - Working principle of a surge protection device SPD
In general, the most effective approach to protect large installations of lighting equipment against surge over-voltages is by cascading multiple protective stages. Each stage combines the necessary balance between discharge capacity and voltage protection level. This way, a first stage (typically a ‘Type 1’ or ‘Type 2’ SPD) provides robustness, thus diverting most of a spike’s energy, while a second stage (typically a ‘Type 2’ or ‘Type 3’ SPD) provides ‘fine’ protection. Thus the peak voltage reaching the equipment always stays below the critical level. Installations have unique characteristics, therefore, SPD solutions should be customized and tailored appropriately with proper lightning protection and grounding systems in place.
Of the causes of surges mentioned in international protection standards, the ones most likely to affect a public lighting system are:
- Direct lightning strikes on distribution lines (conducted through the power lines)
- Lightning strikes near a building/structure (creating induced surges)
The protection solution is installed downstream of the main circuit breaker in the distribution panel circuit board, in parallel to the main system. So it diverts the energy of the surge to earth, limiting the voltage peak to a tolerable level for equipment connected downstream.
To guarantee proper protection of a luminaire, the distance between it and its protector circuit must be as short as possible. If the distance between a protected distribution panel and several luminaires is more than 20 meters, using a second protection stage (of Type 2 or 3) is recommended, even if the protection level of the first stage seems to be sufficient.
Properly made ground connections are essential to the effective functioning of a lightning protection system, and every effort should be made to provide ample contact with the earth. This does not necessarily mean that the resistance of the ground connection should be low, but rather that the distribution of metal in the earth or upon its surface should be such as to permit the dissipation of surge energy.
Figure 4 - Circuit protection solutions for luminaires and lighting distribution panels
Stage 1 Protection: In the most vulnerable environments, the SPDs can be installed on a DIN rail. It protects not only surge over-voltages (max surge current of 40 kA) but also power-frequency over-voltages. Power-frequency over-voltages are often caused by insulator breakdown in circuitry.
Stage 2 Protection: When designing installations, the area should be assessed for its vulnerability to lightning strikes. If the vulnerability is high, additional protection up to 10kV is recommended. In these cases, it is recommended to use an SPD, in addition to the standard protection at luminaire level.
Stage 3 Protection: Standard protection at luminaire level IEC61547 states that all luminaires should be protected from over-voltages up to 1 kV in differential mode and 2 kV in common mode.