What is a seal-in contact?

A normally open contact of the motor starter, wired in parallel with the start button. Once the coil pulls in, this contact closes and keeps the coil on after the start button is released.

Why is the stop button normally closed?

So current flows through it until someone presses it. Pressing it breaks the circuit and drops out the coil.

Electrical drawings: lesson 6 of 6Harder

Read a motor control circuit

One control circuit, five questions about it. Then a new drawing. Get 10 right to finish.

Same drawing, question 1 of 5

Which device is the start button?

How a start/stop circuit works

Press start (normally open). Current flows through stop, start and the overload contact to the coil.
The coil pulls in. Its seal-in contact, in parallel with start, closes.
Release start. Current now flows through the seal-in contact, so the motor keeps running.
Press stop (normally closed), or let the overload trip. The coil drops out, the seal-in contact opens, and the motor stays off until start is pressed again.

Pilot lamps are fed through more contacts of the same starter: a normally open one for RUN, and sometimes a normally closed one for STOPPED. On IEC drawings the terminal numbers help: 13-14 and 23-24 are normally open contacts, 31-32 is normally closed, A1-A2 is the coil and 95-96 is the overload contact.

Questions

What is a seal-in contact?: A normally open contact of the motor starter, wired in parallel with the start button. Once the coil pulls in, this contact closes and keeps the coil on after the start button is released.
Why is the stop button normally closed?: So current flows through it until someone presses it. Pressing it breaks the circuit and drops out the coil.

Explained in our own words and checked against several references. For the full rules, see the NEC (NFPA 70), NFPA 79, IEC 60617, IEC 60445 and IEC 81346-2. Your drawing legend always comes first.