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Home / Technical Articles / The essentials of designing MV/LV single line diagrams (symbols & drawings analysis)

What is the Single Line Diagram?

First of all, power system designers should always communicate their design requirements through a combination of drawings, schedules and technical specifications. One of the key tools in developing and documenting an electrical power system is the Single Line Diagram (shortened SLD).

The essentials of designing MV/LV single line diagrams (symbols & drawings analysis)
The essentials of designing MV/LV single line diagrams (symbols & drawings analysis)

Single line drawing starts with the incoming power source from the utility service and/or on-site generation and their associated distribution equipment. It then follows the power flow down through the various conductors as well as any voltage transformations to feed distribution equipment buses for the key loads served.

Initially, the Single Line Diagram (SLD) provides a framework for the incorporation of different types of required information such as:

  1. Incoming service voltage and utilization voltages required.
  2. Electrical distribution equipment ampacity and short-circuit ratings.
  3. Overcurrent / short-circuit protection.
  4. Conductor types (i.e., cable or busway) and sizes.
  5. Cable lengths (to determine voltage drop)
  6. Transformer kVA sizes, ampacity, impedance and voltages.
  7. Generator kW sizes and voltages.
  8. Motor loads and voltages.
  9. Other power quality equipment such as surge protection devices, power factor correction capacitors or uninterruptible power supplies.

Ok, let’s dive into details of a single line diagram now, by describing the most important elements:

Table of Content:

  1. The Life Of a Single Line Diagram
  2. Importance of Single Line Diagram
  3. Standardized Drawing Symbols
    1. General
    2. Circuit Protective Devices
    3. Distribution transformers
    4. Current Transformers
    5. Voltage Transformers
    6. Protection Relays

1. The Life Of a Single Line Diagram

A Single Line Diagram may start out in the Design Development Phase of a project as a basic concept. Other information can be added throughout the design cycle. It can then be copied and modified to create a number of alternate drawings showing different system approaches.

This permits the power system designer to analyze the impact of each arrangement on cost, redundancy and projected physical space requirements.

The System SLD takes on increasingly more importance as the project evolves through the Design Development Phase. Input from the other architectural, mechanical, plumbing, electrical and fire protection professionals on the design team helps to better define the various equipment loads and develop the power system one-line to accommodate them.

Example of single line diagram
Figure 0 – Example of single line diagram using different colors for cable types

At some point in this stage, a construction manager may be brought in to assist the owner and architect in assessing the design’s constructability.

Various improvements that could increase energy efficiency and/or reduce construction costs are often suggested.

Moving toward the end of the Design Development Phase, the Single Line Diagram and associated drawings such as equipment room elevations and floorplans are provided to the client for their review and approval. At this point, both the client’s comments and the construction manager’s additional inputs are integrated into the design.

This final set of approved design development drawings, which include the Single Line Diagram, are used as the basis for the development of the construction drawings.

Moving into the Construction Document Phase of a project, alterations are made to the Design Development Electrical Drawing set.

At some level of completion (typically 90%), these drawings are sent out to finalize budgetary estimates and narrow the field of contractors to be included in the selection process. During the push from 90% to 100% completion of the construction documents, the construction manager or the general contractor is asked to provide a Guaranteed Maximum Price (GMP).

During the Bid or Negotiation Phase of a project, Bid Document Drawing Sets are sent out to a list of potential contractors. Estimators at these contractors review the Bid Package and tabulate the value of the electrical equipment, conduit and cable costs plus manpower necessary to build out the project.

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2. Importance of the Single Line Diagram

It is important for the power system designer to ensure the Single Line Diagram and other design documents contain as much information as possible, to assure that bidding contractors include all the correct requirements in their pricing. Errors and/or omissions on the construction contract documents can lead to expensive contractor change orders and project cost overruns after the contract is awarded.

During the various stages of a project design, changes are made often to reflect the client’s preferences and budget.

As the design process continues, coordination between the MEP (Mechanical, Electrical and Plumbing) design disciplines become more critical. If the design professionals are not synchronized on these changes, a previously unanticipated piece of equipment may be chosen or added to the project.

As an example, where an engineer had previously allocated a 250 A circuit breaker to feed the anticipated load, as a result of an equipment change, a 400 A breaker must now be provided. The impact of this change can result in a contractor bid that does not include both the correct breaker AND the correct cable sizes to feed the larger load.

Often times, requirements such as electronic trip units or their protective functions such as Long, Short, Instantaneous and Ground (LSIG) or Ground Alarm (LSIA) are not indicated on the Single Line Diagram. This can lead to equipment being supplied with standard thermal-magnetic trip units that may lack the setting capabilities to achieve the proper selective coordination required.

Other requirements such as: Zone Selective Interlocking of breakers, 100% rated breakers, drawout or electrically operated breakers and key interlock schemes can be overlooked if they are not documented on a Single Line Diagram and coordinated in the specifications.

Finally, electrical equipment is subject to environmental issues such as wet areas and may require specific enclosure types to be provided. omenclature on the Single Line Diagram, such as 3R or 4X, adjacent to these items can clarify what enclosure type is to be provided.

The proper use of notes on the Single Line Diagram can further define the requirements.

As an example, a note can be added clarifying that all NEMA 4X rated enclosures are to be of 316 stainless steel versus the less expensive 304 Grade. The difference between these two grades is critical as 316 Stainless is far more resistant to saltwater, sulfuric acid and chlorides, and is preferred in several applications including pharmaceutical manufacturing and wastewater treatment plants.

Example of Notes on Single Line Diagram
Figure 1 – Example of Notes on Single Line Diagram

The System Single Line Diagram is the common map that all the other project documents must reference and be checked against. To ensure consistency and avoid conflicts after a project is awarded to a contractor, distribution panelboard schedules and specifications also need to include the correct information about details such as the enclosure type required.

For these reasons, it is critical that the engineer be vigilant and take a proactive role in identifying changes and updating the Single Line Diagram and associated design documents appropriately and consistently.

The Single Line Diagram in the following paragraphs is an example developed for illustrative purposes only and was developed to show a wide range of product applications. This diagram will be referenced throughout the remainder of this article.

The references to external drawings is for illustration only and not referencing actual documents within this section or elsewhere.

Power System Single Line Diagram
Figure 2a – Power System Single Line Diagram
Power System Single Line Diagram (Continued)
Figure 2b – Power System Single Line Diagram (Continued)

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3. Standardized Drawing Symbols

3.1 General

In the North American market, the American National Standards Institute (or ANSI for short), in cooperation with the Institute of Electrical & Electronics Engineers has developed standardized drawing symbols and nomenclature to represent common devices represented on one-lines, control schematics and other electrical drawings.

The existing Standard for North America (including the Canadian Standard CSA Z99) is IEEE 315-1975 (Reaffirmed 1993)/ANSI Y32.9. This version recognizes that “Electrical diagrams are a factor in international trade: The use of one common symbol language ensures a clear presentation and economical diagram preparation for a variety of users.

Consequently, the Standards Coordinating Committee has added various International Electrotechnical Commission (IEC) symbols that are in use worldwide.

Item A4.1.1 of IEEE 315 defines a Single-Line or (One-Line) Diagram as: “A diagram which shows, by means of single lines and graphic symbols, the course of an electric circuit or system of circuits and the component devices or parts used therein.”

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3.2 Circuit Protective Devices

Components such as those representing circuit protective devices like fuses and circuit breakers are indicated in their most basic form. Device representations can be created by adding other components and nomenclature to the base component drawing.

Low-voltage <1000 V circuit breakers are represented by the first two of the following symbols shown in Figure 3.

Circuit Breaker Symbols
Figure 3 – Circuit Breaker Symbols

Medium-voltage circuit breakers shown on a one-line typically incorporate the Basic Square Breaker symbol with the ANSI Device Number 52 inside. Medium-voltage breakers may be either fixed mount (square with device number inside) or drawout as shown in Figure 3.

It is important to develop a naming convention so personnel working on or responding to an event on the power system can readily identify the equipment experiencing any problems.

This naming convention is also useful for those doing preventive maintenance in documenting which specific switchgear, breaker, transformer or protective relay they need to address.

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3.3 Distribution transformers

Transformers are common components of a power system and are used on both medium-voltage and low-voltage applications to step a voltage up or down to a desired level. They are available in a variety of winding configurations (learn more about connections).

Because there are many types and configurations of transformers available, it is necessary to properly document the specific requirements on the Single Line Diagram.

Primary unit substation transformers are used to convert a medium voltage to another medium voltage. Secondary Unit Substation Transformers transform a Medium Voltage to a Low Voltage Level, generally under 1000 Vac. They are available in Fluid-Filled and Dry-Type styles.

Both types of unit substation transformers can be supplied with fans to increase the transformer’s kVA ratings.

Transformer Information and Symbols
Figure 4 – Transformer Information and Symbols

Transformer information:

  • Transformer “T1”
  • Primary unit substation style
  • 13.8 kV delta primary – 95kV BIL
  • 4.16/2.4 kV grounded wye – 60kV BIL
  • Eaton “peak” 55c/65c/75c
  • 7500/8400/9156 kVA KNAN
  • 9375/10500/11445 kVA KNAF
  • Fr-3 fluid filled, 6.5% minimum z
  • With surge + lightning arresters
  • P/FA = current rating. Primary, forced air.
  • S/FA = current rating. Secondary, forced air.

Figure 4 from the medium-voltage half of the system single line diagram shows “T1” as Eaton “Peak” Style Triple Temperature Rated, 7.5 MVA, FR3 Envirotemp™ Fluid Filled, Power Transformer. The transformer’s kVA ratings are indicated at the KNAN, (Natural Air Cooled by Convection – Over 300° C Fire Point Fluid Filled) and KNAF (Forced Air Cooled Over 300° C Fire Point Fluid Filled) ratings.

The “T1” transformer is described as “Delta” Primary, “Wye” Secondary configuration in the text as well as further depicted by the relationship of the “H1, H2 and H3” connections to the X1, X2, X3 and X0 symbols adjacent to it.

Similarly, the verbiage in the text calls for surge and lightning protection. Symbols for the arrester and the capacitor are shown connected to the incoming terminations. Their actual ratings should be defined on the drawing or in the specifications.

Both the transformer’s primary and secondary amps are included as a reference for sizing the conductors. This is useful to determine the quantity and size of the MV cables per NEC Article 310.60. While medium-voltage conductors are available in 90° C (MV90) or 105° C (MV105) ratings, the actual terminations in the transformer or switchgear cable compartments are limited to 90° C.

When sizing the MV cables, the NEC derating factors must also be applied depending on the type of raceway or duct bank that will be required.

Where higher transformer secondary currents are involved, a busway flange and non-segregated busway can be supplied to connect it to the downstream MV switchgear (as shown in Figure 4). Proper selection and application of the busway requires that the rated short time and short circuit withstand current values be specified.

NOTE! Short-circuit values are critical in the design and specification of all electrical equipment in a power system.

The transformer’s Impedance, (often abbreviated as %Z) must be shown on the Single Line Diagram in order to calculate the required ratings of downstream equipment as indicated in Figure 5.

Incoming Service Calculation
Figure 5 – Incoming Service Calculation

It is important to remember that all transformers designed to ANSI standards have a plus and minus 7.5% tolerance for impedance. If a transformer requires an absolute minimum impedance to ensure the secondary short-circuit level does not exceed a critical value, it must be noted on the SLD and in the accompanying project specifications.

Consideration must also be given to the types of cable terminations based on the available short-circuit ratings. Where the available short-circuit exceeds 12.5 kA, medium-voltage molded rubber dead-front terminations are generally not an option. In these cases, the type of terminations must be specified.

Stress Cone cable terminations are available in either Hot Shrink or Cold Shrink configurations. Porcelain terminators or potheads are a more expensive option, but often have higher short-circuit ratings.

Stress Cone cable termination
Stress Cone cable termination (credit: Simon Dubitsky , George Greshnyakov)

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3.4 Current Transformers

Current transformers are used in both LV and MV applications as sensing devices for protective relays and meters. They are available in “donut” style, which encircle the conductor, as well as bar style, which is bolted in series with the load conductors.

Both styles work on the principal of electromagnetic coupling. A current flowing through the conductor they surround induces a proportional isolated low level signal (either 1 A or 5 A) that can be measured by an electromechanical or electronic device.

Current transformers may be shown in several formats as indicated in Figure 6 below.

The dots, X’s or boxes are used to denote the instantaneous polarity orientation of the CT. The polarity marks on the conductor generally face toward the source of the current flow. The polarity mark on the CT winding represents the relationship of the CT’s X1 secondary terminal to the H1 medium-voltage terminal on bar type CTs or its input orientation for donut style CTs.

Current Transformer Symbols
Figure 6 – Current Transformer Symbols

In the case of Differential Protection Circuits such as the 87-T1 Transformer Differential or the 87-B1 Bus Differential, the CTs are oriented in opposing directions as illustrated in Figure 7 below.  This permits the Differential Relays to measure the current going into a transformer or busbar and deduct the current flowing out of it.

When more current is flowing into the zone of protection than is proportionally flowing out, the relay senses the “differential” and trips the circuit breakers at high speed to protect against a fault anywhere in the zone.

Note the “Y” symbol, as well as the quantity “(3)” next to the CTs. This represents three CTs configured in a three-phase grounded wye arrangement.

While most of the CTs on the system single line diagram in Figure 2a and 2b are shown this way, the CTs on the output side of the 2000 A breaker S1A are not grounded. This is done to indicate to the equipment manufacturer or installing contractor that the CT inputs to the relay should not be grounded in more than one location.

CTs generally are wired to shorting terminal blocks as indicated by the “SB” in the box shown in Figure 7. These are used to short out the secondary of the CTs prior to equipment installation or when servicing them.

Example of Differential Circuit with Current Transformer Symbols
Figure 7 – Example of Differential Circuit with Current Transformer Symbols

It is highly recommended that the design engineer show Test Switches on the System SLD and include them in the specifications. These are shown on the SLD as a box with “TS” in it.

Test switches are used during protective relay testing to provide an alternate path to inject current and voltage from a test set, when commissioning these devices in the field.

When designing a power system, it is necessary to select the ratio and the accuracy class for the CT’s. For protective relaying, the CT must be sized to ensure they do not saturate under fault conditions.

This may result in a higher accuracy class with more physical mass or a higher CT ratio being specified.

Most of the CTs shown on Figure 7 are Standard Accuracy Class for the ratios selected. The exception is the single 600:5 CT in transformer T1’s Neutral to Ground Connection. This is shown as a high accuracy CT.

When selecting CTs for metering purposes, it is best to use the CT ratio as close to the actual load as possible. This is done to increase the accuracy at the low end of the range because the CT’s excitation begins to deteriorate at about 10% of its ratio setting. As an example, a 600:5A fixed ratio CT would begin to lose accuracy at 60 A.

Where loads are light, during construction or during early build out stages, the actual current that must be measured by the meter may be only 100 A. Multi-ratio CTs are frequently used to set the maximum ratio lower. If set at 100:5A, this would improve accuracy down to 10 A for a 100 A load.

Conversely, as the end loads grow, the maximum ratio setting can be easily increased by changing the CT tap settings.

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3.5 Voltage Transformers

Voltage transformers are used to step higher voltages down to safe levels for inputs to relays and meters. Traditionally, voltage transformers (VTs) utilize a higher primary voltage winding that is a fixed ratio to the 120 Vac secondary winding.

Examples shown on the SLD are 14,400 V:120 V (a ratio of 120:1) or 4200 V:120 V (a ratio of 35:1).

Voltage transformers are often referred to as potential transformers or PTs. They are illustrated symbolically as shown in Figure 8.

Voltage Transformer Symbol
Figure 8 – Voltage Transformer Symbol

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3.6 Protection Relays

The secondary output of both voltage and current transformers are measured by protective relays and used in calculations involving preset thresholds.

Voltage monitoring elements of protective relays compare the input from the VTs against a desired set-point to see if the system voltage is over or under that nominal value. If the value exceeds a plus or minus tolerance band around the set-point, an output contact or contacts in the relay change state to signal an alarm or trip the circuit breaker open.

Microprocessor-based relays offer tremendous functionality over the older electromechanical and solidstate predecessors. Many of these devices offer multiple types of voltage and current protective elements.

Protective relay elements are generally denoted by a number or characters as defined in the ANSI/IEEE C37.2 Standard for Device Function Numbers, Acronyms and Contact Designations.

These element numbers are shown in a circle on the SLD. A given relay may have multiple voltage and current elements shown in a common box, such as the for example Eaton’s relay EDR 5000-M1 protecting the 52-M1 breaker in Figure 9.

The numbers in parenthesis define the quantity of each specific element. In many cases this quantity is (3); one for each of the three phases. In some cases, such as the 50/51N function, this is shown as a quantity of (1). The symbol to the right of this relay represents a transition from (3) individual phase elements to a single residual neutral protective element.

The output of each protective function is shown with a dashed line and arrow indicating what action is to be taken if the relay determines the monitored values exceed the preset thresholds.

The protection relay’s 50/51 Elements (Instantaneous Overcurrent and Time Overcurrent respectively) are shown tripping a high-speed 86-M1 Lockout Relay.

The elements of the Eaton’s ETR-5000-T1 Transformer Differential Relay are shown similarly, also tripping an 86-T1 lockout relay.

Protective Relay Element Symbols
Figure 9 – Protective Relay Element Symbols

In both cases, the associated (86) lockout relay then trips the incoming main breaker “M1” and the transformer secondary breaker “S1” (see their positions).

Lockout relays are used to multiply the tripping contacts for a given function so they can be wired into multiple breaker’s separate control circuits as indicated for the 86-B1 device on the System Single Line Diagram. Their primary function, however, is to require a manual reset of the Lockout Relay mechanism by trained personnel after the cause of the fault is determined and corrected.

The 27 and 59 functions shown in the protection relay’s monitor undervoltage and overvoltage respectively. Their outputs are shown combined into a single dashed line directly tripping both the incoming main breaker “M1” and the transformer secondary breaker “S1”.

This reflects the engineer’s desire to have only one output contact for both the 27 and 59 functions. Because two breakers need to be tripped, this will only require two separate relay contacts instead of four individual output contacts otherwise necessary.

The direct trip shown on the System SLD purposely does not use an 86 lockout relay, as this under or over voltage disturbance is anticipated to be caused by the utility and not a fault on the end user’s power system. In these instances, a separate contact from the relay may be allocated to start a backup generator or to initiate a Main-Tie-Main Transfer Scheme.

Protection relays from SLD (EDR-5000 and ETR-5000) are programmable multi-function devices with many protective elements that can be utilized simultaneously. In a more fully developed protection scheme, certain protective elements (such as the 50/51 functions) can be used in both relays to back each other up in the event of a failure.

EDR-5000 Protective Relay Elements Available
Figure 10 – EDR-5000 Protective Relay Elements Available

Figure 10 shows the many protective elements available in the EDR-5000 Feeder Protective Relay. Figure 9 shows some additional important information about the equipment required in the dashed box that comprises utility switchgear “USG-1A”.

This switchgear is defined as 15 kV Class with a 95 kV basic impulse rating. The bus is rated to handle 1200 A even though the actual ampacity flowing through it will be under 500 A. The equipment will be operating at 13.8 kV and have a short-circuit rating of 50 kA Symmetrical.

Because this SLD is for educational purposes, a hypothetical short-circuit value at “Point A” from the Utility is shown for reference at 11.65 kA. In actuality, this value would be part of a short-circuit study.

If using a software to calculate the downstream short-circuit values, the cable lengths and conduit types as well as the transformer impedance would factor into the calculations.

The “USG-1A” switchgear on the SLD is shown with a 50 kA rating when other lower ratings such as 40 kA and 25 kA are available at 15 kV. This has been done as an example to future design engineers who may be involved in urban areas with mediumvoltage services. These MV services typically have higher available shortcircuit capacity.

In most cases, the serving utility may have specific specifications for the switchgear and breakers used as medium-voltage service equipment.

One such requirement is the breaker’s close and latch rating. Where higher fault current values exist, some utilities specify this value at 130 kA peak, which is more in line with the older 1000 MVA rated design. Because the 40 kA K=1 design’s close and latch rating is only 104 kA, the 130 kA close and latch rating for the 50 kA breaker would dictate it being used instead.

It is very important to be cognizant of nuances in all utility specifications to avoid costly problems or delays in energization.

It is always best to follow Occupational Safety and Health Administration (OSHA) approved practices and assume that the circuit is live until a calibrated voltage reading probe attached to a hot-stick determines otherwise.

Most utilities and institutions involved in the distribution of medium-voltage power use portable ground cables that are applied only after no voltage presence has been confirmed. This requires that ground studs be mounted in the switchgear in order to facilitate their OSHA compliant grounding procedure.

As shown on the Single Line Diagram, there are ground studs on the incoming and outgoing sides of both the “USG-1A”, (13.8 kV) and “PSG-1A” (4.16 kV) switchgear. Applying these portable ground cables requires a safe disconnection of power in the zone to be grounded to ensure personnel safety.

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Source: Power distribution systems – Eaton

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


    Aug 26, 2019

    I am an Operator of 110kV Substation in INDIA. I am interested in studying more about EHV substations.your site found to be very useful.

  2. shekhar Salgaonkar
    Jul 30, 2019

    Article was good. Please explain short circuit and current calculations on primary and secondary side .

  3. Engr. Md Anichur Rahman
    Jul 30, 2019

    How to analysis LV/MV SLD by using Etap software? Is it possible?

  4. Pradeepkumar
    Jul 29, 2019

    I am a graduate of electrical and electronics work experience also related to power plant switch gear station.

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