Wiring up a ceiling light with earth cable
From your wire-colours and terms I guess you are in the UK


  The light itself is a 5 arm ceiling light ...  I've never seen a light with an earth wire before


If it has any exposed conductive parts, they need to be earthed. Maybe the arms are metal?


  What am I doing wrong?


You have probably mis-wired the ceiling rose. One of the wires from the switch loop will not be what its colour suggests.

A typical UK ceiling rose is wired like this



A is power in
B is power out (to next light fitting)
C is to/from switch

Note the red tape on the blue wire (position 2) to indicate that this is not a neutral, as the blue would suggest, but a switched live.

Since your breaker is tripped regardless of the switch position, you probably have something other than the obvious error that occurs when the tape is missing. 



You should not have trouble connecting three or four earth wires to the earth terminal. You can use a connector block if that makes it easier but I believe they must be enclosed in a junction box or in the ceiling rose. There isn't much spare room in a ceiling rose.  



The ones on the right are the traditional ones used in the UK, a lighting circuit uses the ones rated 5A. The connectors to the left are Wago 222 and 221 connectors - they make it easier to connect multiple wires together - the leftmost one connects three wires together. There are 2,3 or 5-position versions of each. Wago also make push-in connectors (273 and 773 series) for various numbers of solid-core wires (not stranded). Other manufacturers like Ideal also make them. You can buy these from screwfix and other suppliers.