It's not the same as CSCS, but it is affiliated to CSCS. The Electrotechnical Certification Scheme is specific to electrotechnical occupations and as such is the card that those working in the sector should posess.
you get your ecs test passed which lasts for 2 yrs. so from the date of passing you can get on sites with the ecs certificate showing you've passed.
then just as its coming up to expire apply for your jib card. as you have a valid ecs cert the jib card is issued with ecs (cscs affliated) pass on it which lasts for another 3 years
just like you must have done with a car mot due to expire in 2 weeks but you can still get 1 years road tax with it