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When thinking of surge protection, the first thought is lightning strikes. With 100 strikes per second and 8 million strikes per day worldwide (according to MET office data), lightning is a major hazard to buildings, offices and factories but many of these buildings have lightning conductors to carry the electricity harmlessly to the ground. However, this will still leave some transient or internally generated surges from man-made loads that we need to be aware of and manage. This is where system protection is paramount when looking at keeping downtime to a minimum, avoiding equipment damage and protecting personnel.
Circuit protection is critical to protect a machine from currents greater than the machine or device’s current-carrying capacity. Proper electrical protection is the key to safely removing the effects of dangerous overcurrent due to short circuit, overloads and voltage transients from switching surges and other abnormal conditions.
The main guidance one should take concerning surge, overvoltage and overcurrent protection can be found from the industry standards: surge protection to BS EN 62305 / BS EN 61643; or, within the 17th Edition wiring regulations, Types of SPD / Lightning Protection Zones. Also consider Section 534 – Devices for Protection Against Overvoltage. Ultimately, good wiring practices, grounding, and well-implemented bonding techniques – to protect both machines and personnel – are key to supplemental overcurrent protective devices.
July 2018 saw the publication of the 18th Edition of the Wiring Regulations, set to come into place in January 2019. The new Section 534 is about the installation of the SPDs as required or specified in section 443 or elsewhere. The Lightning Protection Zone (LPZ) concept was featured in the 17th edition but now has far more detail in the 18th. The concept comes from BS EN 62305, the full lightning protection standard and describes the way that SPDs are deployed within the installation.
Before we can start to look at the protection of our ‘on machine’ sensitive devices we need to ensure that the large external surges (25Kv+) are reduced on the income to the building. The Type 1 SPD at the origin is with the main incomer, the Type 2 SPD would be with the SDBs (Sub Distribution Board) where the MCBs are located and lastly the Type 3 SPD could be at plug top fuse protection levels or control panels.
With the increasing dependence on technology in almost every business, protecting sensitive equipment is essential. In some industries, downtime can lead to losses of millions of pounds every hour.
Interference-free operation, therefore, is crucial and the integrity of these systems has to be maintained even under relatively extreme circumstances such as thunderstorms. And while safety devices such as fuses will protect against excess current, they are ineffective against the high voltage transients and short-duration spikes that lightning can generate on power supply lines.
Most system designers incorporate electrical circuits with branch circuit devices, such as an appropriately labelled circuit breaker or fuse. These devices protect against fire and electrical shock by limiting current flowing through wires and provide a means to remove electrical power during equipment service.
This is an area that is often overlooked or not fully understood by system designers. Branch circuit protection does not necessarily protect a load such as a power supply, a PC, or a programmable logic controller (PLC). To provide this protection, Type 3 SPDs provide additional equipment protection whether branch circuit protection is already provided or not required. For process control, SPDs must withstand the harsh environments common in process applications, and do so in ever-smaller packages as the space in a cabinet decreases. And, as the available space is reduced and other requirements become more demanding, SPDs are constantly changing to meet these new challenges.
This, of course, is all fairly obvious and forms the basis of the lightning protection systems that are incorporated into many buildings. However, despite the clear dangers and the critical nature of many systems, the high volume of insurance claims for lightning damage indicates that many building operators are failing to ensure appropriate lightning measures are taken.
To promote a comprehensive approach to overvoltage control, the emphasis is on covering all cables transiting a zonal interface. This ensures that natural events such as lightning are dealt with as well as the internally generated surges from man-made loads. The standard also guides the designer to look at the practicalities of the installation and performance parameters within the distribution board and closer to the sensitive end loads. So using a combination of Type 1, 2 and 3 SPDs will ensure complete security and protection to maintain process production and profit.
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