What does normally open mean?

The contact is open at rest: no current flows until the button is pressed or the coil is energized. A normally closed contact is the opposite: it conducts at rest and opens when operated.

How do I read contacts in series and parallel?

Contacts in series must all be closed for current to flow (AND). Contacts in parallel need only one closed path (OR).

Electrical drawings: lesson 2 of 6

Contacts and control logic

Work out which buttons make the lamp light. Get 10 right to finish.

Which of these makes the lamp light?

Three rules

Normally open (NO) closes when pressed. Normally closed (NC) opens when pressed.
Series: every contact in the line must be closed.
Parallel: one closed branch is enough.

Follow the current from one rail to the other. On a US ladder diagram the rails are on the left (L1) and right (L2). On an IEC drawing they run along the top and bottom. The load, here a lamp, sits at the end of the path.

Example

A normally closed PB1 in series with a normally open PB2. Press nothing: PB1 is closed but PB2 is open, so the lamp is off. Press PB2 only: both are closed, so the lamp lights. Press both: PB1 opens, so the lamp is off.

Questions

What does normally open mean?: The contact is open at rest: no current flows until the button is pressed or the coil is energized. A normally closed contact is the opposite: it conducts at rest and opens when operated.
How do I read contacts in series and parallel?: Contacts in series must all be closed for current to flow (AND). Contacts in parallel need only one closed path (OR).

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.