London Fire Hydrants Carry Colour-Coded Discs

According to the local newspaper, The London Free Press, the colour-coded, reflective discs hanging on all fire hydrants in London indicate how how much water a firefighter can expect when attaching a hose to the hydrant. Blue indicates the highest flow rate, 95 litres a second, and red the lowest at less than 31 litres. Green and orange marked hydrants fall in the middle.

A hydrant’s water flow depends on the available pressure, and the size and condition of the water line. While water pressure typically depends on elevation, other factors can affect the flow. Where time counts, this information helps firefighters decide what size of hose and what pump settings to use.

For more info on this, I did a blog post some years ago looking at the shape of the discs. Many believe these discs are shaped like a Maltese Cross. If you are familiar with the Maltese Cross, you will know that this can't be true. The discs are totally the wrong shape: curved and not spikey. Here is a link to my post on the Florian Cross shape used to make these discs and why it is a suitable shape.