[Bored chemist]

How do they do it for power lines ?

Often by ... coating the lines in plastic.

With some power lines they use air as the insulator but that's not a problem. To get electrolytic corrosion, bot polarities ( + and - ) must be in the same body of water.

as well as that, they also use Al for some overhead which doesn't have the same corosion problems as the steel in bridges.

They generally use a combination of aluminium and steel, which provides its own set of challenges.

[Peter Dow (OP)]

What is the corrosion danger from electrically charging a bridge structure ?

A reasonable concern shared by the Bridge Manager of the Queensferry Crossing, Jason Cheetham who wrote to me stating

"We would be concerned that running an electrical current through the wires would increase the
possibility of hydrogen being emitted within the sheath, with the potential to lead to broken wires."

HDPE Sheathed Stay Cable Strands
Other than at the anchorages where it is stripped off so the wedges can grip onto the wires, the strands are protected by an HDPE sheath with good electrical insulator properties – more than enough to insulate the cable heating voltages required – a maximum of about 100V for the longest cables.


No Cathodic Hydrogen Charging
The integrity of the HDPE sheath prevents a corrosion process whereby moisture from the outside air could in theory penetrate any wear-and-tear gaps in the strand’s HDPE sheath to react slowly with the zinc galvanic coating to liberate hydrogen atoms from the water which could threaten to embrittle the steel wires over time with the potential eventually to lead to broken wires.

Likewise, the same integrity of the HDPE sheath would also prevent any possibility of the establishment of a cathodic hydrogen charging circuit through non-existing electrolyte-infiltrated gaps in the sheath.

How do they do it for power lines ?

So in normal operation, the heating voltages and currents would be unable to emit hydrogen within the sheath – for the same reason that domestic electrical power cables do not emit hydrogen within their insulating sheaths either.

However, should there ever be a failure of the HDPE sheath integrity and consequential ingress of damp, salty air (or any other electrolyte) which establishes a new fault or leakage circuit then the heating system’s fault or leakage current detection circuits must be able to detect the fault condition, immediately cut the power to avoid any risk of exacerbating mechanisms of hydrogen emission and embrittlement and report the fault condition so as to prompt remedial maintenance, of course.

DC Circuit Diagrams
Locating all the electrics at the deck anchorages, while leaving the strands earthed at the tower anchorages, offers advantages for design, development, installation, commissioning and servicing.




Heating strands pair voltage balance detector

The window detector circuit compares the isolated power supply’s potential with respect to earth to detect the expected balance of resistance and voltage in the heating strands pair. If an imbalance fault develops then the safety switch is used to cut the power.

[Peter Dow (OP)]

and the rest of the bridge isn't connected.

The rest of the bridge will be earthed and you should be able to see from my circuit diagram that I have connected the earth to all the heating circuits at the tower anchorages.

So the rest of the bridge is indeed connected via the earth and that's worth remembering because the power supplies and power lines at the deck anchorages must be kept isolated from the earth for the heating circuits to work properly and for safety reasons. One can't afford to ignore the risk for someone at the deck to touch a power line and touch the bridge earth otherwise the heating voltage may decide it is easier to go through the person than through the cables! Recognising that risk is why I have designed this safety switch which will cut the power if there is any imbalance in the 2 voltages dropped over each of the strand pair which carry the heating current.