When placing a cable between two equipment locations with separate grounds having a potential difference of 2.1 V rms, you should avoid using UTP (Unshielded Twisted Pair) cables. UTP does not provide any shielding and therefore cannot handle the voltage difference safely. It is prone to noise and could introduce ground loops. On the other hand, both multimode and singlemode fiber optic cables are ideal since they use light to transmit data and are immune to electrical interference. STP (Shielded Twisted Pair) provides shielding which helps reduce the impact of the potential difference and noise, but it requires proper grounding to be effective. Therefore, UTP should not be used in this scenario.