The convention
Two PDUs, two utility feeds, two power supplies per server: that is what "A/B" means, and it only protects you while every server really is on both. The convention on this site and in most data centres we supply is red for the A feed and blue for the B feed. There is no standard that mandates it; it is a habit that works because red and blue are far apart on the eye, both read under amber emergency lighting, and neither is the black that every other cord in the room already is.
How to colour-code a rack
- Decide which PDU is A and which is B, and label the PDUs themselves, top and bottom.
- Every cord from PDU A is red; every cord from PDU B is blue. No exceptions inside one rack, even for the switch that "does not matter".
- Single-corded equipment goes on an automatic transfer switch fed from both PDUs, and the one cord from the ATS to the device is a third colour (or black) so nobody mistakes it for a redundant pair.
- Walk the aisle when you are done. If a rack shows only one colour at any server, that server is not redundant, whatever the drawing says.
Where it pays off
During maintenance. A technician shutting down PDU B for a swap can see every cord that is about to lose power without tracing anything, and can see which servers will stay up on red. The locking mechanisms on this site keep cords in; the colours keep people from pulling the wrong one on purpose.
Buying by colour
Every family here comes in red and blue. Open a family and choose the colour, or let the Lock Finder filter for you: red P-Lock, blue P-Lock, red equipment-side locks, blue equipment-side locks. Other colours (orange, yellow, green, white) are for sites that code by something else, such as tenant or voltage; the same discipline applies.