Electric signal

How to read it

Signal lines join the bubbles to each other and to the valves they move. In Kuphaldt’s Lessons In Industrial Instrumentation, a dashed line out of a transmitter means the signal is electric rather than pneumatic, most likely the 4 to 20 milliamp DC standard. The DOE’s handbook draws instrument signals as dashed lines too, among the line symbols that tell piping, instrument signals and electrical wires apart. Course notes by Peter Woolf and others call it the electrical impulse line.

Kuphaldt notes that the 2009 edition of ISA 5.1 dropped the triple-slash line, an older alternative for electric signals, and the single backslash that once marked an on/off signal; plain signal lines can now carry either kind.

Also searched as: electric signal p&id symbol, electric signal symbol.

Look-alikes

A line with pairs of short slashes is a pneumatic signal. The DOE’s chart also gives a dashed line as an alternative for electrical wiring, so a legend sheet settles which is meant.