24V - 48V standard for lab. tips
The voltage limit for safety is 50VDC. 

There is no difference between 48VDC and 24VDC in terms of safety and standards. 

Below 50VDC is considered safe because the impedance of the body and the current flowing through the body at this voltage makes it unlikely for someone to be injured.

If you want to protect against injury, you can add a differential breaker (residual current device)