In itself, a ground potential rise is not harmful to any equipment or person bonded to the same ground potential. However, where a conductor (such as a metallic telecommunication line) bonded to a remote ground potential (such as the Central Office/Exchange) enters the area subject to GPR, the conflicting difference of potentials can create significant risks. High voltage can damage equipment and present a danger to personnel. To protect wired communication and control circuits in sub stations, protective devices must be applied. Isolation devices prevent the transfer of potential into or out of the GPR area. This protects equipment and personnel who might otherwise be exposed simultaneously to both ground potentials, and also prevents high voltages and currents propagating towards the telephone company's central office or other users connected to the same network. Circuits may be isolated by transformers or by non-conductive fiber optic couplings. (Surge arresting devices such as carbon blocks or gas-tube shunts to ground do not isolate the circuit but divert high voltage currents from the protected circuit to the local ground. This type of protection will not fully protect against the hazards of GPR where the danger is from a remote ground on the same circuit.)