Electrical Distribution System in a Data Center: Medium-voltage (MV) and Low-voltage (LV) switchgear

So far we explored the Electrical Distribution System in a Data Center. The critical supply from the utility / transformer / generator to the load is enabled by various types of power equipment. One of the key components of the critical supply is switchgear.


Medium-voltage (MV) Switchgear

Medium-voltage switchgear is generally located in the power substation of large-capacity data centers (i.e. greater than 1 MW IT load). This equipment is typically fed directly from the utility and usually marks the utility service entrance to the building.

If a MV generator is present, it also feeds the MV switchgear. The following figure shows an example of MV switchgear single line diagram:-

10kV MV Switchgear Single Line Diagram

Other than simply distributing power, the MV switchgear is responsible for disconnecting faults and controlling the MV power distribution system, for example, when isolating a redundant section for maintenance.

Basically medium-voltage switchgear is the assembly of four cubicles as shown in the Figure above: two incoming units (incomers) or main section, outgoing units or feeder section, voltage metering unit, and bus section or tie section/breaker.

The outgoing unit distributes three-phase power to the primary (upstream) side of the MV / LV transformer. Due to safety distances at MV, typically each cubical is limited to only one MV circuit breaker.


The following are some typical electrical parameters for MV switchgear. Values for these parameters vary according to local regulations:


Voltage ratings:-

Two key voltage ratings for MV switchgear are rated voltage and rated lightning impulse voltage (equivalent to the ANSI basic impulse level i.e. BIL). For example, an ANSI MV switchgear solution may have a rated voltage of 15kV and 95kV BIL (i.e. impulse voltage).


Current ratings:-

The rated current of MV switchgear is always specified by the manufacturer. Another key current rating is the rated short-circuit withstand current similar to the North American National Electric Code’s (NEC) short-circuit current rating (SCCR).

For example, an ANSI MV switchgear solution may have a rated current of 1200A and 40kA SCCR.



Low-voltage (LV) Switchgear

Typically LV switchgear / switchboard is located in the electrical room and marks the utility service entrance for data centers less than 1 MW.

An example of LV switchgear is shown in figures below. If a LV generator is used, the generator would feed the LV switchgear. Apart from distributing power, the LV switchgear is responsible for disconnecting faults and controlling the LV power distribution system.

Note that in the case of a medium voltage generator, this transferring function occurs at the medium voltage switchgear level.

In real situation, the single line diagram will look something like this:-

A device known as an Automatic Transfer Switch (ATS) has traditionally been used to switch between utility and generator. However, the current trend is to have LV breakers perform this function in lieu of the ATS device. Also, there is a practice of using PLC logic for switching between utility and generator sources.

The following devices are always assembled in LV switchgear: horizontal busbar, vertical busbar, circuit breakers, meters, switches, surge arresters, relays, etc.

LV switchgear / switchboard installed in a data center is typically a combination of some of the following functional units: incoming feeder from the secondary side of the MV / LV transformer or LV generators, power control center (PCC, i.e. for downstream UPS), motor control center (MCC, i.e. for pumps), the power factor correction/harmonic filtering and the bus connections.


The following are key electrical parameters of the LV switchgear. The values for these parameters vary according to local regulations:


Voltage ratings:-

Two key voltage ratings for LV switchgear are rated voltage and rated lightning impulse voltage. ANSI does not specify the impulse voltage for LV switchgear. For example, an IEC LV switchboard solution may have a rated voltage of 690V and 12kV rated impulse withstand voltage.


Current ratings:-

The rated current of LV switchgear is always specified by the manufacturer. Another key current rating is the rated short-circuit withstand current similar to the North American National Electric Code’s (NEC) short-circuit current rating (SCCR).

For example, an IEC switchgear solution may have a rated current of 5000A and 85kA rated short-circuit withstand current.

About Us

SMA combines with professional Chartered Engineers (CEng) from the Institute of Engineering Technology (IET), the Chartered Institute of Building Services Engineers (CIBSE) and the Hong Kong Institution of Engineers (HKIE). Our engineers have more than 20 years experience in data center design & build, building services engineering and energy conservation in the private and public sectors.

The team prepares the engineers and IT personnel to face any challenges in data centers and critical facilities of any size, in any location. For other design considerations / topics in data center and critical infrastructure, please visit 

(1) Site Selection,
(2) Space Planning,
(3) Cooling,
(4) Redundancy,
(5) Fire Suppression,
(6) Meet Me Rooms,
(7) UPS Selection,
(8) Raised Floor,
(9) Code & Standards,
(10) Transformers and Harmonic Distortion,
(11) Multi-mode UPS Systems,(12) Electrical Rooms,
(13) Generator Systems,
(14) Generator Fuel Systems,
(15) Battery Systems,
(16) Earthing / Grounding and Bonding, etc.