The universal P&ID symbol for a level switch is a circle tagged LS (Level Switch) connected to a vessel line, while electrical schematics use a standard limit-switch symbol modified with a float, tuning fork, or paddle actuator. Your governing standard depends entirely on your region and drawing type: ISA-5.1 dictates process diagrams in the US and globally, while IEC 60617 and JIC govern the electrical panel schematics.

Below is the direct reference for interpreting, drawing, and wiring level switches, followed by the exact decision path to select the right physical sensor for your application.

The Complete Level Switch Symbol Reference Table

Read this table by matching the drawing type you are looking at. If you are reading a Piping and Instrumentation Diagram (P&ID), look at the ISA Tag column. If you are wiring a control panel, look at the Electrical Schematic column.

Function / Tag ISA P&ID Symbol & Tag IEC / JIC Electrical Schematic Practical Meaning & Application
Level Switch High (LSH) Circle with 'LSH' inside, linked to vessel top via solid/dashed line. Standard NO/NC contact symbol with a float actuator pointing UP. Triggers when liquid rises to a specific high point. Used to start a drain pump or close an inlet valve.
Level Switch Low (LSL) Circle with 'LSL' inside, linked to vessel bottom. Standard NO/NC contact symbol with a float actuator pointing DOWN. Triggers when liquid drops to a low point. Used to stop a pump (dry-run protection) or open an inlet valve.
Level Switch High-High (LSHH) Circle with 'LSHH', often flagged with a red triangle or 'A' for alarm. Independent secondary contact symbol, usually wired to a safety PLC or hardwired relay. Critical high-level safety trip. Acts as a backup if the primary LSH fails or the control loop is overwhelmed.
Level Switch Low-Low (LSLL) Circle with 'LSLL', flagged for critical low alarm. Independent secondary contact, wired to emergency stop circuit. Critical low-level safety trip. Prevents pump cavitation or boiler dry-firing.
Level Transmitter (LT) Circle with 'LT', often with a square root symbol if measuring flow via head pressure. 4-20mA analog signal line (not a discrete switch contact). Provides continuous level measurement (e.g., 0-100%), not a discrete on/off trip point.

Regional and Standard Variants: Which Applies to You?

Level switch symbols are not universal across all drawing types. You must apply the correct standard based on the document in front of you and your geographic location.

Rule of Thumb: P&IDs use ISA-5.1 globally (with minor regional tag tweaks). Electrical ladder logic and panel schematics use IEC 60617 in Europe/UK/AU, and JIC/NEMA standards in North America.
  • ISA-5.1 (US & Global Process): Uses letter combinations (L for Level, S for Switch, H/L for High/Low). The symbol is always a circle (or a circle inside a square if it's a shared/displayed DCS function). The physical actuator type (float, ultrasonic, capacitive) is not shown on the P&ID symbol; it is only specified in the instrument datasheet.
  • IEC 60617 (Europe & Global Electrical): Focuses on the electrical contact. A level switch is drawn as a standard mechanical switch contact (NO or NC) with a specific actuator symbol attached. A float actuator looks like a small circle with a line through it; a tuning fork looks like a 'U' shape.
  • JIC / NEMA (US Industrial Panels): Similar to IEC but uses distinct line-weight and contact notation (e.g., a limit switch contact is drawn with a specific 'wobble' line). The actuator is explicitly drawn as a float (a circle with a crosshair) or a paddle.
  • Old UK (BS 1646 / Legacy): You may encounter legacy British Standard symbols in older plants, which used a simple rectangle with 'LS' inside and diagonal lines to denote the physical linkage. Treat these as obsolete and map them to modern IEC equivalents during panel retrofits.

Symbols and Rows People Get Wrong

Misinterpreting a level switch symbol or its physical wiring state is a leading cause of flooded basements and burnt-out pump motors. Watch out for these specific traps.

1. The "Normal" State Confusion (NO vs NC)

In electrical schematics, Normally Open (NO) and Normally Closed (NC) refer to the switch's state when it is unactuated (the resting state). For a bottom-mounted sump pump float switch hanging freely in an empty pit, the unactuated state is dry/empty.

The Mistake: Hobbyists and junior techs often assume "Normal" means the "Normal operating level" (i.e., full of water). They wire a pump to a NO contact expecting it to turn on when the water rises, but because the switch is designed to close when the float rises, the pump never starts. Always verify if the schematic defines 'Normal' as dry or submerged.

2. Confusing LS (Switch) with LT (Transmitter)

A Level Switch (LS) provides a discrete digital signal (0V/24V or dry contact). A Level Transmitter (LT) provides a continuous analog signal (4-20mA or 0-10V). If your P&ID shows an 'LT' but you wire a simple float switch to a PLC analog input card, the PLC will read a floating or maxed-out value, and the control loop will fail.

3. Safe Interpretation of Faded or Missing Markings

Physical float switches (like the ubiquitous MAC3 or SJE SignalMaster) are often installed in harsh, humid environments where the stamped NO/NC and wire color codes fade or peel off entirely.

Never guess the wiring of an unmarked level switch on a mains-voltage circuit. If the markings are faded, de-energize the circuit, lock out the breaker, and disconnect the switch wires. Use a multimeter in continuity mode (the beep setting). Manually flip the float upside down. The two pins that show continuity (beep) when the float is inverted represent your closed circuit state. Map this to your schematic before re-energizing.

Decision Path: Choosing the Right Physical Level Switch

Schematics tell you how to wire it, but the process conditions dictate what to buy. Use this decision tree to terminate on a specific, reliable sensor type for your application.

IF your process condition is... AND the environment is... THEN pick this technology & concrete model
Clean water, sewage, or sump pumping Non-hazardous, standard pit or tank PVC Tethered Float Switch. Pick the SJE SignalMaster or MAC3. Cheap ($20-$40), reliable, handles minor debris.
Turbulent, sloshing, or aerated liquids Vessel with mixers or boiling action Tuning Fork with Time Delay. Pick the Endress+Hauser Liquiphant FTL31. Ignores bubbles and turbulence; no moving parts to jam ($400-$600).
Viscous, coating, or corrosive chemicals Liquid that builds up on sensors (e.g., paint, resin, acids) Capacitive Level Switch. Pick the IFM LK1024 or a PTFE-coated probe. Measures dielectric changes through a non-metallic tank wall or coated probe ($150-$300).
Bulk solids (grain, plastic pellets, cement) Dusty silos or hoppers Rotating Paddle Switch. Pick the Bindicator Mini-Safe or UWT Rotonivo. Mechanical torque stops the motor when buried in solids ($250-$450).
Flammable solvents or explosive dust Class I Div 1 / ATEX Zone 0 or 20 Intrinsically Safe (IS) Tuning Fork. Pick the Endress+Hauser FTL51 paired with a Pepperl+Fuchs IS barrier. Requires certified IS wiring practices ($800+).

Wiring, Grounding, and Safety Caveats

When moving from the schematic to the physical wiring, electrical safety and code compliance override the drawing.

  • Mains Voltage Sump Pumps: If your level switch is directly switching a 120V or 240V sump pump motor, the switch must be rated for the motor's Full Load Amps (FLA) and Locked Rotor Amps (LRA). Most cheap float switches are only rated for 10A-13A resistive. For a 1/2 HP pump, use the float switch to trigger a definite-purpose contactor or a motor-rated relay, not the motor directly. Furthermore, NEC Article 210.8 requires GFCI protection for 125V/250V receptacles in damp locations like basements and crawlspaces.
  • Low Voltage (24VDC) Control Circuits: When wiring a 24VDC level switch to a PLC input, always use shielded, twisted-pair cable if the run exceeds 10 feet or passes near VFDs (Variable Frequency Drives). Ground the shield at one end only (usually the panel/PLC end) to prevent ground loops from inducing false high-level trips.
  • Intrinsically Safe (IS) Installations: If your P&ID shows a blue tag or an 'IS' notation next to the level switch symbol in a hazardous area, you cannot use standard NM-B or THHN wire. You must use IS-rated cable (often blue-jacketed) and route it with a minimum 50mm separation from non-IS power cables, or separated by a grounded metal barrier.

By matching the correct P&ID tag to the physical reality of your process, and verifying the unactuated state with a multimeter before applying power, you eliminate the most common failure points in liquid and solid level control.