An electrical connector comprising a socket for mutually engaging with a header to provide electrical connection for one or more conductors of the socket to one or more conductors of the header, the socket comprising a socket housing configured to hold one or more socket-side conducting terminals each of which is adapted and arranged mutually to engage with a corresponding header-side conducting terminal to provide electrical connection to the one or more conductors of the socket, and the header comprising a header housing configured to hold one or more of the header-side conducting terminals mounted within the housing to provide electrical connection to the one or more conductors of the header, each of the one or more of the header-side conducting terminals being configured mutually to engage with a corresponding one of the one or more socket-side conducting terminals when the socket is mated with the header, wherein the housing (122, 132) of one of the socket or the header includes a first locking arm flexibly connected to the housing (122, 132), the first locking arm including a first projection having a first cam surface, the first projection being arranged to project into a space between the first locking arm and an adjacent surface of the housing and the adjacent housing surface includes a second projection having a second cam surface, the second projection being arranged to project into the space between the first locking arm and the housing surface towards the first projection, and the housing (122, 132) of the other of the socket and the header includes a second locking arm comprising a head having first and second engaging surfaces (722, 726), which are configured forward with respect to a first recess and a second recess formed in the second locking arm, the first engaging surface being configured to engage with the first cam surface of the first projection of the first locking arm, and the second engaging surface being configured to engage with the second cam surface of the second projection as the socket and the header are being mated causing the first locking arm to be displaced from a closed position against a biasing force provided by the flexible connection of the first locking arm to the housing (122, 132) to allow the head of the second locking arm to pass into the space between the first locking arm and the adjacent housing surface, and after the head of the second locking arm has moved into the space between the first locking arm and the housing surface, the first projection on the first locking arm can be received in the first recess on the second locking arm, and the second projection can be received in the second recess formed in the second locking arm so that the first locking arm can return to the closed position, characterized in that the second locking arm is resiliently connected to the housing (122, 132), the resilient connection allowing the head of the second locking arm to be displaced in a plane substantially perpendicular to a plane of the first locking surface so that the second locking surface can be displaced away from the housing adjacent surface to dis-engage from the first locking surface, and the first projection includes a third cam surface and the head of the second locking arm includes a third engaging surface on an adjacent side to the first engaging surface, which engages with the third cam surface as the head of the second locking arm is displaced away from the housing adjacent surface, the third cam surface and the third engaging surface displacing the first locking arm as the second locking arm is displaced away from the housing adjacent surface and retracted from the first locking arm with the second locking surface displaced to be clear from the first locking surface allowing the second locking arm to be retracted from the first locking arm.