Earthquake protection for electrical infrastructure

30 March 2017

In areas known to be prone to earthquakes, buildings and technical infrastructure need to be earthquake-resistant. This also applies to electrical equipment within those buildings. In this article, Hartmut Paul of Rittal looks at the concept of earthquake-resistance in relation to electrical infrastructure and describes his company’s approach to designing and testing earthquake-resistant equipment and systems.

Natural forces have always posed a threat to humans and repeatedly cause casualties and extensive damage. Whilst extreme meteorological occurrences, such as floods and heat waves, can be forecast relatively accurately these days, earthquakes remain relatively difficult to predict.

Earthquakes cause more fatalities than other natural disasters, as well as structural damage to buildings and transportation infrastructure. They can also cause serious damage to buildings’ internal systems.

When it comes to earthquake protection, building safety is often the primary concern. This is certainly understandable, as primary earthquake damage to buildings generally causes the most fatalities, and the majority of damage to property also occurs this way. However, technical infrastructure installations such as electrical switchgear systems and data centres also require effective protection against earthquakes. This not only applies to critical systems, such as those in power plants or production facilities in the chemical industry, but may also be important for other users.

Intensities, magnitudes and earthquake zones

In terms of physics, an earthquake is a shock wave that emanates from an epicentre below ground. This shock wave causes the Earth’s crust to vibrate with a complex frequency spectrum, both horizontally and vertically. This is described in terms of the relating amplitudes and frequencies on the Earth’s surface. Because the energy released by an earthquake cannot be measured directly, we use various scales to describe the strength of an earthquake. This includes differentiating between scales of intensity and scales of magnitude.

Subjective intensity scales

Intensity scales are based on the macroscopic effects of an earthquake – such as the severity of damage to buildings – and the subjective impressions of the people who feel or hear the earthquake. A frequently used scale to indicate the intensity is the Mercalli scale, which was developed in 1902 and is still in use today (See Table 1). Scales of intensity can only be used to a limited extent in sparsely populated areas where there are few buildings where damage might occur and not many people to report their experiences.

Measuring technology for earthquakes

Scales of magnitude are based on measurements taken by seismometers, which measure the local vibrations in the Earth’s surface in terms of speed, acceleration and displacements. Calculations using these measurements can indicate the strength of the earthquake. The best-known scale of magnitude is the Richter scale, which was developed in the 1930s and is still used today for this purpose.

Table 1: Mercalli Scale compared to the Japanese JMA Scale

The probability of an earthquake is important for risk assessment

Besides categorising earthquakes according to strength, i.e. their intensity or magnitude, another important aspect is the probability that an earthquake of a given strength will occur. Earthquake risk zones are defined in order to facilitate a reasonable risk assessment. For instance, there are five earthquake zones in the United States. Zone 0 is where strong earthquakes can be practically ruled out, whereas earthquakes of quite a large magnitude are relatively probable in zone 4. Parts of California are typical examples of earthquake zone 4. The requirements relating to earthquake protection for IT and telecommunications installations, and for electrical infrastructure installations, are often based on these earthquake zones.

However, zoning varies from country to country. In most countries in Europe (including Germany), the zones range from 0 to 3; but up to five zones might be categorised. This is further complicated by the lack of uniformity between the national standards that define the zones (See table 2). Thus it is not possible to draw a simple comparison between national standards at face value. However, classifying earthquake zones is important when it comes to delivering earthquake-resistant systems, as the appropriate measures – incurring additional costs – need only be taken in high-risk areas.

Earthquake damage to electrical infrastructure

In order to evaluate the relevance of earthquake protection for electrical installations, an overview must first be established of the damage that can potentially occur in an earthquake, including any consequential damage that might arise if an electrical system fails. Damage to buildings is usually the focus of attention following an earthquake. Depending on the kind of building involved, the values for the systems installed in it are often higher than for the structural elements themselves. It makes sense, therefore, to look beyond the earthquake-resistance of the building alone and to also consider the potential requirements regarding its systems in the case of an earthquake.

Malfunction or structural damage

It is particularly important that the installations of critical, safety-relevant infrastructures, such as in nuclear installations, remain operational even after high-magnitude earthquakes. This calls for a very extensive range of measures, which are beyond the scope of this article. A high level of systems availability, and thus robust protection against earthquakes, is also particularly vital for telecommunications and IT. At the same time, the ability of installations to remain operational for a given time or to resume service quickly are also important issues following an earthquake.

The frequency of the vibrations that occur during an earthquake generally ranges between 0.3 Hz and 50 Hz. The stresses these vibrations exert on a switchgear system can cause both malfunctions and structural damage to the entire system. Malfunctions can be remedied with little delay, so a switchgear system can be put back into service relatively quickly following an earthquake. This might typically involve a loose contact or temporary short-circuit that is interrupted by the installation’s safety systems.

More disruptive damage might include the dislocation of components from an enclosure’s support rail or mounting plate. Serious damage to the switchgear system generally causes a protracted interruption to the energy supply – say, if the earthquake were to move an enclosure, perhaps dislodging it from its anchoring or even tipping it over.

Table 2: Soil acceleration in Europe and in the USA

This category also includes any structural damage to the enclosure. Enclosures play an extremely important role because if they cannot withstand the earthquake, the entire system is going to fail. This makes earthquake protection for enclosures a key factor in all the relevant standards. However, this aspect can never be considered in isolation, but rather the surrounding building and all the installed components must also comply with the relevant requirements as well. Thus a suitable enclosure alone will not suffice if the system needs to remain operational after, or even during, an earthquake. In addition, the installed components must also meet the requirements of the appropriate standard and the functioning of the entire system must be proved under test conditions.

Overview of current standards

Earthquake protection is important from various points of view. Correspondingly, different standards are applied depending on the technical discipline concerned. These standards can be roughly divided into the categories of civil engineering, IT and telecommunications, and electrical engineering. Furthermore, the applicable standards also differ according to the geographical location of the market. Building standards play a role in the construction of earthquake-resistant electrical switchgear systems but they are not described in great detail here. However, as electrical systems are for the most part installed in buildings, these standards also play a certain role, particularly the floor mounting of the enclosure.

Some of the standards that apply to electrical engineering, IT and telecommunications have their origin in a building standard but transfer into specific requirements for related equipment. For instance, buildings can actually amplify the effect of an earthquake, potentially further increasing the amplitude and accelerations that are exerted on an electrical system.

Relevant standards for equipment

Three specific sets of standards are particularly relevant to switchgear systems and other electrical engineering and information technology infrastructures – DIN EN/IEC 60068-3-3, IEEE 693 and Telcordia GR-63-CORE. Other standards, for example from the construction industry, are generally not relevant to switchgear production - it can be assumed that structural engineers will comply with these standards. Only the interface between construction and electrical engineering – where the building and enclosure meet – plays a certain role. This means that references to the respective building standards also appear in various standards.

DIN EN/IEC 60068-3-3

IEC 60068-3-3, identical to DIN EN 60068-3-3 [Beu93] in Germany and Europe, is primarily a guideline for checking electrical devices for seismic resistance. The standard distinguishes between a general and a specific seismic class. The specific seismic class should be used where knowledge of the seismic movement exists based on the local geographical situation or the building in which the device is to be installed.

IEEE 693

The 693 [IEE05] standard issued by the Institute of Electrical and Electronics Engineers (IEEE) specifies the parameters for earthquake-resistant switchgear. It defines the test methods for complete switchgear systems and for individual components such as power circuit-breakers. Besides test methods, the standard also contains guidelines for designing earthquake-resistant switchgear in relation to buildings, foundations and fixing enclosures to the floor. Thus reference is also made to building standards in this document.

Telcordia GR-63-CORE

Although the GR-63-CORE [Tel02] generic requirements originally developed by Bellcore – now Telcordia – for telecommunications are not a formal standard as such, they form a very commonly stipulated requirement in contracts, especially in the United States. The basic premise is that systems – such as data centres – need to offer a high level of structural resilience to various influences such as moisture, fire, pollutants and earthquakes. The document refers to the designated zones in the United States (zones 0 to 4), where zone 0 represents a very low risk and zone 4 a high risk of earthquakes (See Table 2). In order to ensure high system availability, the requirements in zone 4 are correspondingly higher than in zone 0. These requirements are very high and can therefore also be found in other standards (e.g. IEC 60068-3-3, DIN EN 61587-5 (RRS for uniaxial acceleration), ETSI EN 300019-1-3).

Typical test methods

In general, there are two possibilities when testing systems for earthquake-prone areas. The first is to test the complete switchgear system with the desired configuration - including all components installed in the enclosures- in a suitable laboratory. This option is quite costly and time-consuming and, according to Rittal’s experience, is only usually required by a small number of users.

The second option involves the control or switchgear manufacturer testing components (including enclosures) for earthquake-prone areas. For this proof, Rittal tested its TS 8 type enclosures in approved laboratories according to a defined test method.

The following paragraphs describe a typical testing method to determine earthquake resistance of enclosures.

All the applicable standards prescribe tests on a vibrating table to demonstrate an enclosure’s earthquake resistance. The aim is to simulate the effects of an earthquake – i.e. vibrations and shock stresses – in the laboratory. The test piece is mounted onto the vibrating table, where it is subjected to a pre-defined test program. The test piece must not incur any structural damage as a result, meaning that no load-bearing parts may be damaged, warped, broken, etc. In addition, no important connections should separate. The same also generally applies for enclosure doors, hinges and locks. Functional tests are then performed – to ensure that the system still functions as required post-stress.

Different frequency spectra in the standards

The precise test requirements vary according to the standard in question, particularly regarding the exact frequency spectrum and related accelerations. The test method specified in Telcordia GR-63-CORE is outlined here as an example. First, the enclosure is mounted on a vibrating table and fitted with acceleration and path recorders in its centre and on top.

During the test, the enclosure must be loaded to an extent that simulates a real installation.

Acceleration-Frequency-Diagram

The tests are then conducted by running a specified program that reaches the defined acceleration values at frequencies of between 1 Hz and 50 Hz (the so called Required Response Spectrum (RSS)). The incoming spectrum, the so called Transmission Response Spectrum (TRS), on the test piece is the key. This depends on the way the test is set up and on the mass and geometry of the test piece. The displacements of the test piece, measured in its centre and at its top, must not be greater than 75 mm (3 inches) at any point during the test.

The vibration table tests described above are performed in all three dimensions. The RRS prescribed in GR-63-CORE lasts 31 seconds in each dimension. The loads exerted on the enclosure during these tests correspond approximately with those experienced during an earthquake with a strength of 8.3 on the moment-magnitude scale.

Very similar test spectra

In order to compare the requirements of the vibration table tests in all three relevant standards, the required RRS can be plotted in an acceleration-frequency diagram (fig. 2).

This reveals that the spectra applied in the individual standards are quite similar but have different acceleration values in the relevant areas. It also becomes apparent that zone-4 certification according to GR-63-CORE almost completely meets the requirements of the other two standards.

Earthquake-resistant enclosures

One method of constructing stable mechanical buildings able to withstand tremors, in use since ancient times, is by using half-timbered methods. This uses poles that are bound together at the ends to form triangles. As a result, the forces exerted on the poles are almost exclusively compression or tensile forces, which make the entire construction extremely sturdy and also reduce its weight. This principle has proved a successful way to construct houses, bridges and other load-bearing structures. These days, the norm is to provide buildings with as rigid a structure as possible, for example using solid concrete constructions.

Another approach, which is particularly prevalent in high-rise buildings, is to use a pendulum as an active element. An earthquake’s movements make the pendulum swing, which absorbs almost all of the energy so that no damage occurs to the building’s structures. A third possibility decouples the object to be protected from the earth (and earthquake) by means of a system that absorbs the forces. When a building or piece of equipment is not firmly fixed to the ground but instead stands on dampers, it is shielded from most of an earthquake’s accelerations. In this case it is necessary that the weight to be dampened is exactly known and there must be sufficient spring/damping deflection available.

Earthquake kit – enclosure extensions

Rittal uses the half-timber principle to make its TS 8 enclosure earthquake-resistant. The unit is robust and exceptionally flexible and optional extra earthquake accessories stiffen the construction of the enclosure frame to comply with even the strict zone-4 requirements of GR-63-CORE. The earthquake kit consists of stays that are screwed into the sides of the enclosure’s frame to significantly increase its rigidity. Gusset plates in the corners of the frame add extra stability. The mounting plate is also more rigid and screwed to the stays.

Earthquake extension accessories for TS 8 enclosure

A more stable plinth is also available for fixing the enclosure to the floor for earthquake protection.

Three sizes of the TS 8 standard enclosure were tested for conformity with the zone-4 requirements of GR-63-CORE by the independent body EQE International Ltd at the University of Bristol and IABG in Ottobrunn.

When fitted with the special earthquake accessories (consisting of earthquake kit, earthquake plinth and comfort handle), the enclosure achieved zone-4 certification with contents weighing up to 500 kg on its mounting plate.

In addition, the TS 8 was certified to meet the requirements of zone-3 GR-63-CORE without any special accessories, up to a maximum weight of 152 kg.

The tests were performed on three sizes of TS 8 enclosures and the certification extends to all of these, but the engineering principle behind the frame structure is the same for other sizes, so earthquake resistance can be assumed as long as the following criteria are met:

· The earthquake standard / the frequency spectrum is comparable

· Even weight distribution

· Mounted weight of components equal or less than the tested variants

· Base area equal or bigger than the tested variants

· Height w/o plinth not bigger than 2000mm (or rather the centre of mass higher than 1000mm)

Specific Approach

Even armed with this information about earthquakes, potential damage and the standards that exist, the question remains as to how to respond to a specific inquiry about earthquake-resistant switchgear. First, the geographical location needs to be clarified – for which country and which earthquake zone is the switchgear required? Then the relevant standard needs to be identified to suit this requirement. All requirements relating to the building or anchoring of the switchgear within the building must be taken into consideration by the parties responsible for the construction and structural design.

Structural integrity or functionality

In respect of the above-cited examples of application standards, a distinction has to be made between structural integrity and maintaining the function of a system. If the sole concern is structural integrity, then a certified enclosure, such as the TS 8 enclosure fitted with the appropriate earthquake accessories, as described in the previous section, will generally suffice.

Continuous functioning of a system, as is sometimes required, cannot be guaranteed by an enclosure, of course. Extensive tests are required to ascertain this aspect. Finite-element method structural calculations also can be made in advance. It is important to test the switchgear with the contents that are actually going to be fitted. The distribution and weight of the installed components can influence the vibration behaviour of the enclosure. The way the enclosure is installed – whether on mounting plates, top hat rails or variable bus bar systems – can also affect its behaviour. For this requirement it is therefore advisable to test earthquake-resistant enclosures for each specific scenario, in other words with the intended contents in place.

This article is adapted from the white paper ‘Earthquake protection for switchgear’ available at www.rittal.com/uk-en/content/eng/unternehmen/presse/pressemeldungen/pressemeldung _detail_51072.jsp

About the author

Hartmut Paul has worked for Rittal since 1998 in various departments and is currently responsible for the pre-development of Rittal's enclosure systems. After working on his thesis, he has had a keen interest in the dynamic loads placed on enclosures with a particular focus on earthquake resistance. He has a degree in mechanical engineering from the Duale Hochschule, Mannheim.