EN ISO 13850, 4.1.1 states that the emergency stop function shall be available and operational at all times. What's more, the emergency stop function shall override all other functions and operations in all operating modes of the machine. Any facilities designed to release trapped persons must not be impaired.
In the case of a plug-in control device, if it is removed then the emergency stop must be bypassed using a shorting plug. By using a key switch, the emergency stop can also be bypassed in operation. However, this is not permitted (EN ISO 13850 4.1.1).
When emergency stop devices can be disconnected (e.g. portable teaching pendants) or when machinery can be partially isolated, care should taken to avoid confusion between active and inactive control devices.
In the case of plug-in control devices with an emergency stop (red on yellow background), care must be taken to ensure that there cannot be any confusion between active and inactive emergency stops when the control device is plugged in. This means that the control device must be removed from the machine and locked away. If this is not possible, an emergency stop (red on yellow background) may not be used. The emergency stop must then be defined as a rapid stop, for example, in another colour (e.g. black or grey).
If a press is involved, this is described specifically in the C standards EN 692 and EN 693.