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Surge Protection Methodology
 
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Sensitive critical process equipment is deemed to be at greatest risk and should be assessed for surge protection based on Points 4, 5 and 6 of the 6 Point Plan, as detailed in the Site Audit Methodology within the Technical section of this site.

The greatest risk to any critical operational and electronic process management systems is through the service entry of unshielded, buried or exposed conductors that may enter (or exit) a building or structure, or which may travel and interconnect between different buildings/structures.

Such cablings will include:
Incoming and outgoing mains power cabling
Incoming and outgoing telecoms and extension cabling
LAN and serial data
Coaxial CCTV
RTU telemetry cabling
4-20ma signaling and
Various instrumentation cabling.

Lightning does not discriminate in its treatment of any long metallic conductor that is exposed to its direct and indirect coupled energy and EMF, and it is the exposure to these indirect effects of magnetic induction, capacitive coupling, and ground potential rise, (not withstanding the lesser prevalent direct strike risk) which causes the greatest risk for the incidence of lightning related damage and disruption.

Point of Entry/Point of Exit

To deal with this risk, a sites engineering and operations management team need to look at a holistic regime so as to isolate their most critical process equipment from these directly connected and exposed cablings, where the most effective way to manage this, is through a basic and simple protection philosophy, that we refer to as “Point of Entry/Point of Exit”.

The Point of Entry, and Point of Exit, involves individually auditing and considering the risk that all incoming and outgoing copper conductors place on the connected equipment when these cables enter, or leave a structure, and where these cablings are shown to present a direct connection to internal critical systems, these cablings should then be fitted with suitably rated SPD’s (Surge Protection Devices) installed at the cablings point of entry, and/or point of exit. The presence of any installed SPD at Point of Entry, therefore delineates that a protected zone has been created, where any incoming surge current will be shunted to earth, prior to it entering the facilities.

Strategically, the Main Switchboard, the various levels of Distribution, and possibly even Sub Distribution boards, offer the most effective and convenient placement for SPD’s to be installed on incoming mains cabling, which when considered methodically, provides a sequenced protection regime which provides transient control to all downstream connected critical systems.

At large decentralized sites where there are larger numbers of remote outbuildings, there will most likely also be larger numbers of downstream Distribution and Sub Distribution Boards involved, which will all interconnected by long and often buried lengths of sub mains cabling. Where these cablings are run remotely from a remote main switchboard, this can additionally present an elevated risk from the induced effects of localized lightning activity, to the connected downstream facilities, irrespective that the upstream incoming mains may be surge protected at the Main Switchboard. The reasons for this is that magnetic induction, capacitive coupling and ground potential rise, can occur anywhere on a site, therefore transients may be induced or coupled downstream from Main Switchboard installed surge diverters.

Where these sub mains cables are buried, or are run in PVC underground conduits, this plays no role in any shielding effect to these cablings, hence there will still a requirement to protect these cables at their imminent downstream “POE” (Point of Entry) at the relevant outbuilding or equipment cubicle. The installation of additional set of SPD’s at the outbuildings main Distribution Board, or remote equipment control cubicle , then delineates a further protected and clean zone to which all connected equipment within and connected from those outbuildings if then protected.

What is trying to achieve effectively involves the creation of clean zones where sensitive critical systems are located. POE is all about the creation of protective zones, created by the appropriate placement of SPD’s.

POE also has similar application on those other incoming cabling services, the most common of which will be incoming telecommunications cablings and any outgoing extension lines. From a POE perspective, the Telecoms MDF and IDF termination frames, provides the most strategic location for the installation of these telecoms line SPD’s .

Where some organizations come unstuck is that these MDF and IDF Frames must be earthed, and to simply install SPD’s into these frames which are not earthed, the protection devices require this earth reference, and any installed protection device will not operate correctly without the earth connected.

Diverters versus Filters

Whilst Primary Surge Diverters offer good point of entry protection by removing much of the brunt of lightning related transients, there will still be high residual voltages which remain, which may still be of sufficient magnitude to still damage the more sensitive semi conductor based equipment.

The installation of dedicated Surge Filters at those more critical sensitive loads further reduces the voltages to within the tolerance of sensitive equipment, whilst also reducing the high rates of change associated with voltage and current rise (the dV/dT and dI/dT). These high rates of change can be just as damaging as the over-voltage, yet simple surge diverters do not address this particular aspect.

Such sensitive loads will generally include:

Mainframe Computing and Server equipment
PC and workstations
PABX
Programmable Logic Controllers
Irrigation Controllers
Radio Telemetry Equipment

The following illustration shows the typical let through voltage at the output terminal of a surge diverter, and surge filter when subjected to an identical Cat B ( 6 kV, 3kA @8/20us) test impulse.

This illustration shows that 6000 volts is impressed on the Surge Protective Devices with an associated current of 3000 amps, which reaches 90% of its peak within 8 microseconds, and which decrease to 10 % of its peak within 20 microseconds.

The residual voltage left over from a surge diverter ( shown in green) has been reduced from 6000 volts down to 600 volts, whilst the Surge Reduction Filter (Shown in Red) has slowed down the rate of voltage rise and outputs a much reduced let through voltage with a peak of only 240 volts.

Shunt vs Series voltage levels
Shunt vs. Series voltage levels

As the surge diverter cannot address this rate of change issue, it therefore only offers a coarse level of protection. The surge filter (which includes an additional LC wired circuit in its configuration) slows down these high rates of dI/dT and dV/dT, ensuring a much lower residual voltage is presented, and we refer to these Surge Filters as offering a Fine level of protection and voltage control.

When we discuss “a staged and sequenced protection regime”, we are effectively talking about taking the main brunt of lightning related over-voltage away with Point of Entry Surge Diverters, and using downstream Surge filters which take the pre-clamped voltage waveform, and then reduce this let through down to even safer levels, which are within the over-voltage tolerance of the connected equipment.

POE compliance can effectively be automatically included on any future upgrade projects, by tightening internal engineering and tender specifications, drawings and documents , that may be provided to external equipment suppliers such as switchboard manufacturers, as well as UPS, PLC, Telemetry, IT and Communications equipment vendors.

It is recommended that ANY proposed Surge protection equipment be UL listed, that is the equipment has been independently tested and put through rigorous safety and other testing regimes to ensure the products actually meet their stated specification and performance criteria. Surge Protection devices in particular should be UL listed and hold compliance to UL 1449 Edition 2 and IEC 61643.

Australian Standard AS1768-2007 recognizes the requirement for UL testing in ensuring that any Surge Protective device which may fail due to certain fault conditions,( such as Temporary Over Voltage( TOV)) fails in such a manner so as to not cause a fire.

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