Spark plug manufacturers use different numbers to denote heat range of their spark plugs. Perilex plugs and sockets are 5-pin 3-phase connectors. Schuko plugs and sockets are symmetric AC connectors. A connector assembly consists of an adapter and two connector plugs. These plugs and sockets are usually IP56 rated if fitted correctly. I plugs and sockets from different countries have different pin lengths. Hz and also permitting interchangeable plugs and sockets. Different standard systems of plugs and sockets are used around the world. All use sequential fuel injection with two spark plugs per cylinder. Ignition is realized by two spark plugs. It is port fuel injected with platinum-tipped spark plugs. CE marking is neither applicable nor permitted on plugs and sockets. Existing copper connections are maintained with existing plugs being used. Type-A plugs and receptacles are designed to interoperate. As a metal, yttrium is used on the electrodes of some high-performance spark plugs.