48V DC Telecoms Equipment Disconnection Time

Traditional 'plain old telephone system or POTS' 48V wiring does not really have a disconnection time, as the system is current limited, and a shorted line can be supported for ever (in fact current flow is how the exchange knows the phone is 'off hook' ) 

There are a great many telecom related things (power over ethernet comes to mind as a modern one) that have continued this current limited rather than ADS safety philosophy. 

There is no shock hazard disconnection requirement in BS76771 that sets a rapid disconnection time for voltages that are below LV, and count as ELV 50V AC / 120V DC. 

There are however other standards that will apply, such as the BT SINs and various IEEE standards and of course any maker's instructions.

What is the system you are considering supplying ?

Mike.

Where is this listed in BS7671, is it 411.8 as this doesn't specify DC?

48V d.c. isn't RLV, but ELV. Disconnection times for ADS are per table 41.1 - which for ≤120V d.c. says no disconnection needed for protection against shock. (Basically ≤120V d.c.  is treated pretty much live ≤50V a.c. - i.e. in normal dry conditions is considered safe to touch).

You probably still need to protect conductors (or rather their insulation) from physical damage in the case of shorts and overloads - but the timing of that is much more open (5s is often used as a maximum for dead shorts, just to simplify the calculations, as things are mostly still adiabatic in that range).

   - Andy.