 The Taylor/GRS electric power frame system used a feature called "dynamic indication" where the back EMF generated when the electric motor reached the limit of travel would signal the interlocking logic that the points had finished moving, but not the position of the points on an ongoing basis.This and other open loop systems designed in the 19th and early 20th centuries to save on costly relays, were replaced by closed loop systems after a number of accidents.In North America this is known as "Switch-Signal" protection and any change in the position of a field appliance will immediately set the electric signals controlled by a power frame to danger.