What do the red letters on telephone poles mean?

Anyone who has ever wandered down the street has probably noticed the subtle sign language hiding in plain sight that dots our towns and cities: a code of letters and numbers that elude the uninitiated.

If you live in NSW and have ever wondered what the red and green letters tacked onto street poles mean, you’re not alone.

They relate to nearby fire hydrants and are used to inform emergency crews about where to find important infrastructure when they arrive on the scene.

Professor Stuart Khan from the University of New South Wales took it upon himself to inform the curious public about what the different letters mean on social media this week.

“Have you ever wondered what these little "H", "HP", "HR" and "SV" signs, nailed or adhered to NSW telegraph poles or street sign poles mean?” he wrote on Twitter.

As he went on to explain, the ‘H’ marks the location of a subsurface fire hydrant. It is typically accompanied by a ‘P’ which means it can be found under a nearby pathway or footpath, or an ‘R’ which means it is located under the road.

“Sometimes there are numbers between the letters. The top number is the distance (in metres) from the marker to the hydrant. The bottom number is the size (in millimetres) of the water main,” he explained.

The letters typically face the direction of the hydrant. And if the colour is green it lets fire crews know that they are getting warmer and to keep looking for the red signage on the other side.

A black horizontal line through the middle of the marker indicates the hydrant is located across the road.

In residential areas, hydrants are positioned approximately 50 to 100 metres apart depending on the lay of the land and provision of other services such as phone lines, power and gas, according to NSW Fire & Rescue.

While all states and territories have similar systems, in NSW hydrants are located just a couple of feet underground and are covered by a surface fitting marked with an ‘H’ and sometimes painted yellow to make them easier to see.

While the red letters are known as “primary markers” there are secondary markers such as white or yellow triangles or arrows painted on the road, or blue reflective markers that are easier to see at night.

There are also red ‘SV’ signs which stands for sluice valve, which is an on-off tap fitted to a water pipe.

“The water utility can use it to isolate sections of the water supply for maintenance or to shut off major leaks,” Prof Khan explained.

For some it might have seemed obvious, but plenty of people were grateful for the Twitter thread.

“For all urban obsessives; fascinating thread about minutiae of city sign language. Always saw them, never knew what they were,” wrote one Twitter user.

Others thanked the professor for the fun facts.

“Mate I've been wondering what these were for 7 years ... Thanks!” one wrote.

So next time you spot them, you know what they all mean. And if you ever come across a hydrant hatch which has become obscured in some way, authorities urge you to report it so they can ensure they are kept maintained and easy to find.

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