Learn Spanish (Latin America): El telar es peligroso cuando está abierto.

This sentence perfectly contrasts the two verbs for "to be". We use es (from ser) for peligroso because being a hazard is presented as an inherent characteristic of this machine. We use está (from estar) for abierto because being open is a temporary physical state or condition.

If this were an impersonal warning sign, how could we use an infinitive here?

Since this lesson covers impersonal instructions on signs, a related warning might say No abrir el telar (Do not open the loom) or Mantener cerrado (Keep closed). These independent infinitives act as general commands to anyone reading the sign without addressing a specific "you".

Why is está in the indicative mood instead of the subjunctive after cuando?

We use the indicative after cuando when stating a general fact or a habitual action, which is exactly what this safety warning is doing. If we were referring to a specific future event that hasn't happened yet (like "It will be dangerous when it is opened tomorrow"), we would use the subjunctive: cuando esté abierto.

Is the word telar (loom) related to tela (fabric)?