Wednesday, March 21, 2018

Data Center Design Consideration: Electrical Rooms (1)

Switchboards, switchgear, transformers, generators, and UPSs require space for installation, maintenance, heat dissipation and future expansion (if possible). The wiring, busways, and raceways that distribute the electrical power must be accounted for now or in the future.

Electrical engineers should coordinate with mechanical engineers, architects, structural engineers, and others involved in the design of electrical rooms. Documentation and monitoring of electrical system’s equipment and how it connects to the rest of the facility must be accurately maintained.

We are going to explain the applicable code requirements and evaluate the design criteria for appropriate electrical-room size to accommodate present and future needs. Furthermore, we analyze the requirements for coordinating with structural, architectural, fire protection, and HVAC requirements.


   


Electrical rooms and mechanical, electrical, and plumbing (MEP) spaces are often an afterthought when it comes to building design and planning, either relegated to locations that are left over or deemed undesirable for other planning purposes. This shortsightedness can have unfortunate consequences on the cost, operations, and flexibility of the systems for the future.

NFPA 70: National Electrical Code (NEC) dictates the minimum amount of space needed around the equipment for access, operations, safety reasons, and conduit installation. Together, with the actual equipment sizes, this defines the overall minimum dimensional requirements of the room.





There are three types of general interior electrical spaces that factor into new building design: (1) main equipment rooms, (2) distribution pathways, and (3) local/branch equipment rooms. Code-required working space and dedicated space needs must be met. This article will outline important considerations for these spaces in the early stages of building design as they relate to building type, intended occupancy, size, and future expectations of both the building and the electrical systems.


Working Spaces


Let’s see the different between working and dedicated space as stated by the NEC (see picture below). The working space helps safeguard a clear working zone around all equipment and ensures protection for any workers or occupants within the room. This includes defining minimum width, depth, and height requirements for the working space, which varies due to voltage and the specific equipment. The higher the voltage of the equipment, the greater the depth of the working space. The width should be equal to the width of the equipment and no less than 30 in., while allowing for opening any doors or hinged panels to a full 90 deg. The height should be 6 ft 6 in. from the floor, or the height of the equipment if greater than 6 ft 6 in.

The style and construction type of the electrical equipment dictates whether only front access is required, or if rear and/or side access also is required. For each point of access to a piece of equipment, the minimum working clearances must be provided.



The dedicated space (depicted in red) and the working clearances (depicted in blue) are shown in a new emergency distribution room.


Dedicated Spaces


Dedicated space is a zone above the electrical equipment. It’s reserved to provide future access to the electrical equipment, protection of the electrical equipment from foreign systems, and for installing conduit/other raceways supporting incoming and outgoing circuits. The requirement for dedicated space applies primarily to switchgear, switchboards, panelboards, and motor control centers. The space should be equal in width and depth to the equipment size and extend from the floor to a height of 6 ft above the equipment (or to a structural ceiling, whichever is lower). No equipment or systems foreign to the electrical installation are allowed in this zone by the NEC.



The medium-voltage switchgear sections and unit substation transformers in a large data center installation require additional space and clearances.


The area above the dedicated space may contain foreign systems, provided proper protection prevents damage from drips, leaks, or breaks in these systems. However, it’s good practice to avoid having these systems installed in electrical rooms altogether.

While installations of equipment greater than 1,000 V generally follow the same principles, some of the specifics vary, requiring additional clearance around the equipment due to the increased hazard that these voltages impose (see picture above). Access to this equipment is preferably limited to only those deemed qualified to be there. For this reason, electrical equipment should be installed in rooms or spaces that are dedicated for that purpose and have controlled access.


Main Equipment Rooms


The main electrical room, or service entrance space, should coordinate with the local electrical utility. For example, main equipment rooms have requirements that dictate access to the space from the exterior for servicing, maintenance, and service feeder installation. The type of equipment installed will also further determine the room requirements. The service entrance room is typically located on an exterior wall for both code and practical reasons; it makes installation easier and minimizes the length of the service entrance conductors. Because the service conductors are usually the largest in the facility, this can have a substantial impact on cost.

Using arc-resistant switchgear will also impact space needs. This equipment will be taller and may have a larger footprint. Engineers will also need to account for the potential exhaust gases and arc flash energy by providing a pathway to expel them and relieve the pressure buildup from inside the switchgear.

If an exterior transformer is used to provide the service to a building, feeders from the transformer enter the building and transition to the main service entrance disconnect, typically a switchgear, switchboard, or panelboard. These feeders are often routed underground into the building through the exterior foundation wall via a coordinated opening. Additional coordination with the structural engineer is needed to avoid footings.

The elevation of the service entrance conduits many times do not naturally align with the equipment to which it is routed. Additional space in the form of increased height or footprint commonly is required to allow for the successful transition and termination of these conduits and conductors. Service installations that require medium-voltage equipment and/or transformers installed indoors will require additional elements including more space, higher fire ratings of the rooms (per NEC Article 450), and increased ventilation.

The location of any exterior equipment also needs to be coordinated with other architectural and landscaping elements. Minimum separation distances are often dictated by local codes/ordinances or utility requirements for proximity to screen walls, fencing, vegetation, paths of egress, or building fenestration.

Generator installations offer additional challenges when it comes to defining space needs. Noise, odor, and vibration factor into the location of this equipment within a building. The equipment should be located to minimize disturbances to building occupants and adjacent properties. Many jurisdictions have specific requirements on noise emissions, which will impact equipment placement and other components needed to meet requirements. Increasing the distance of this equipment from sensitive areas is one way of dealing with the concerns, but this comes with added feeder costs and may prove to be more costly than other options.

Sound attenuation and equipment required to meet specific emissions requirements, such as diesel oxidation catalyst, particulate filters, urea tanks, and selective catalytic reduction units, have significant cost implications and require a large amount of space to install.

Tier 4 versus Tier 2 compliance is usually dictated by an owner’s desire to use a generator for utility peak shaving or other non-critical proposes. It is crucial to have a clear understanding of current and future implications in both of these areas from the outset of a project and to discuss them thoroughly with the building owner.

The weight of a generator and the vibration experienced during its operation will have an impact on the building’s structural design. Generators require a lot of ventilation for cooling and combustion needs; getting air into and out of the room is critical and will impact placement.

With regard to fuel storage, most installations require a volume of fuel that dictates an external fuel tank with interconnecting fuel lines. NFPA limits the overall capacity of diesel fuel inside buildings to 660 gal. The relationship of the exterior tank and the generator is also important to minimize pumping requirements and allow for gravity-drain return-fuel piping. This requires the fuel tank to be lower in elevation than the generator.

Direct access to the outside is preferable for maintenance and testing. All of this requires close coordination with the architectural, structural, and mechanical disciplines.

NFPA 110: Standard for Emergency and Standby Power Systems requires the emergency power supply for Level 1 installations to be installed in a separate room, separated from the rest of the building by 2-hour fire-rated construction. While NFPA 110 does allow the emergency power supply system equipment (EPSS; equipment consists of all components from the emergency power supply, or EPS, to the load terminals of the transfer switches) to be installed in the same room as the EPS, it is good practice to keep these separated to help enhance system resiliency. EPS rooms are also prone to additional dust, moisture, temperature fluctuations, and excessive noise during operation that limits the ability to have a conversation and may have a negative impact on other equipment if co-located.

For mission-critical facilities (e.g., financial institutions, data centers, and airports) and other highly sensitive installations, the use of a dry-type, pre-action, or another type of fire protection system that does not rely on a normally wet piping installation is highly recommended. In cold climates, this has an added advantage of preventing pipes from freezing, rupturing, and potentially flooding the EPS room.


Continue Reading: Part (2) - Data Center Design Consideration: Electrical Rooms


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