Honda-Tech - Honda Forum Discussion - what is a fail safe relay? HELP

so i brought in my civic to a honda dealership a few weeks ago to scan my ABS codes because my ABS light was on . One of the codes came up 'battery voltage failure' and my two rear speed sensors were failing. I was told that it was unusual for both speed sensors to fail at the same time so they told me to replace the battery and that should resolve the problem. So i replaced the battery on friday and thought all was good because the abs light went out but yesterday on my drive to school, it appeared again. Now, the abs light keeps going on sometimes and then it goes off sometimes.

These were the following ABS CODES:
61-01 Battery Voltage Failure
54-01 Fail-safe Relay Failure
15-01 Right rear wheel speed sensor open shot
16-01 Right rear wheel speed sensor failure
18-01 Left rear wheel speed sensor failure

so i'm just wondering what the Fail-safe relay is? and where is it?
and if anyone can help me with my problem..thanks!