A loop drawing is the single source of truth for one signal: from the field device, through junction boxes and marshalling, to the I/O channel and its terminal numbers. Our E&I loop drawing service covers new design, modification of existing wiring termination, and keeping the drawing set updated as the panel evolves. Here are the three loop types we draw and test most often.
Example 1 — Analog I/O loop (4–20 mA)
A pressure transmitter loop is the classic case: a two-wire 4–20 mA device powered from the analog input card. The loop drawing shows the transmitter tag, cable and core numbers, junction box terminals, barrier or isolator where required, marshalling terminals, and the AI channel address. Testing follows the drawing exactly — inject 4, 12, and 20 mA at the field end and confirm 0/50/100% at the PLC or flow computer, proving every termination on the sheet.
Example 2 — Digital I/O loop
Digital loops cover status and command signals: a pump running contact (DI) or a solenoid valve command (DO). The drawing records contact type (NO/NC), wetting voltage, interposing relays, and fuse references. Loop testing is a simple but disciplined exercise — operate the field contact or force the output, and watch the correct channel change state, confirming polarity and fail-safe behaviour along the way.
Example 3 — Frequency / pulse I/O loop
Turbine meters and PD meters produce frequency signals, often from a magnetic pickup through a preamplifier. The loop drawing for a pulse loop carries extra detail: shielding and earthing (single-point, to avoid noise counting as flow), preamp power, signal levels, and the high-speed counter channel. We test with a signal generator stepped across the meter's frequency range, confirming the counter reads exactly the pulses injected — because on a fiscal meter, every pulse is money.
Integration and as-built discipline
When panels are modified, we update the affected loop sheets the same day the wiring changes, so the drawing set never drifts from reality. A loop drawing that matches the panel is what makes the next fault findable in minutes instead of hours.