Can stray currents and cathodic protection cause ignition in hazardous areas?

Yes. Pipelines, tanks, rails and steel structures often carry currents they were never meant to carry: the protective current of a cathodic protection system, return currents of electric railways and welding sets, earth fault currents and induced currents. When such a conductor is opened, bridged or connected, for example by removing a spool piece, cutting a pipe or connecting a loading arm to a ship, the current can jump the gap as a spark. Even at a potential difference of well under one volt, the current available from these large, low resistance circuits can make that spark incendive.

Short answer

• It is one of the 13 ignition sources of EN 1127-1 ("stray electric currents, cathodic corrosion protection").
• The danger is the break or the bridge, not the voltage. TRGS 723 notes that separating, connecting or bridging such parts can ignite an explosive atmosphere even at small potential differences, and that the current can also heat poor joints.
• Main controls: bond conductive parts into the equipotential bonding, fit a bonding bridge across the gap before opening a pipe or valve, and keep cathodically protected sections electrically separated from the plant by insulating flanges or joints.
• Insulating flanges belong outside the hazardous area where possible; otherwise TRGS 723 calls for explosion protected spark gaps across them (except in Zones 2 and 22) where dangerous overvoltages are possible.
• Ship to shore: tanker and terminal guidance (ISGOTT) relies on an insulating flange or one electrically discontinuous hose in each transfer line, not on a ship to shore bonding cable.

Where stray and cathodic protection currents come from

TRGS 723, the German technical rule on avoiding ignition of explosive mixtures, lists four sources:

Cathodic protection deliberately drives a DC current through the soil onto a buried pipeline or tank bottom to stop corrosion. The protected structure is therefore an "active" conductor. TRGS 723 states that it generally cannot simply be included in the plant equipotential bonding (local systems excepted), and must be permanently insulated against accidental bridging.

The jetty case: ship, shore and galvanic currents

A ship and a jetty are two large steel structures in seawater, each often with its own cathodic protection. A technical paper on insulation flanges hosted on the website of SIGTTO (the society of gas tanker and terminal operators) describes them as forming a very large battery with very low internal resistance, with typical galvanic potentials of about 200 to 700 mV. A metal loading arm or hose that connects them closes that circuit; the moment it is connected or disconnected, an arc can form at the flange, right where vapour is most likely.

The paper summarises the ISGOTT guidance and adds its own recommendations:

• fit an insulating flange in each loading arm or hose string, or use one single length of electrically discontinuous hose;
• the ISGOTT figures it quotes (from the 5th edition) are more than 10 kΩ for an insulating flange as installed (it may fall to about 1 kΩ in service) and at least 25 kΩ flange to flange for a discontinuous hose, measured with a 500 V tester; check the current ISGOTT edition;
• do not rely on a ship to shore bonding cable: it gives no real protection against these low voltage, high current circuits, and an arc forms if it is accidentally detached or broken;
• switching off the impressed current cathodic protection is not a substitute, because it may take many hours for the potential to fall;
• treat static electricity as a separate hazard, controlled by bonding and earthing on each side.

Pipelines and tank farms: how to work safely

1. Bond before you break. Before removing a valve, meter or spool, fit a bonding cable of adequate cross-section across the gap and leave it until the line is closed again. TRGS 723 requires such bridges before opening and closing connections of conductive plant parts.
2. Know where the insulating joints are. Mark them on drawings and in the field; never bridge them with tools, scaffolding or temporary hoses.
3. Protect insulating joints. Locate them outside hazardous areas where possible; inside (except in Zones 2 and 22), TRGS 723 calls for explosion protected spark gaps with a suitable response voltage where dangerous overvoltages are possible.
4. Coordinate with the cathodic protection operator before cutting into a protected pipeline, and include the work in the permit to work and the ignition hazard assessment.
5. Keep welding return leads close to the weld. Never return welding current through process pipework or tank structures.

In Zones 2 and 22, TRGS 723 allows additional bonding against stray currents to be omitted in general, unless arcs or sparks from such currents occur in normal operation. In Zones 1 and 21 and above, the controls must be in place, and in Zones 0 and 20 internal cathodic protection of metal parts is allowed only if the anodes stay permanently under liquid or the system is switched off and bonded when the level falls too low.

Common mistakes

• Assuming a low voltage is harmless. The spark energy comes from current in a low resistance loop, not from a high voltage.
• Using a ship to shore bonding cable instead of an insulating flange.
• Bridging an insulating flange with a metal hose, scaffold tube or instrument line, which defeats the cathodic protection and can create a spark path.
• Opening a flange "just for a minute" without a bonding bridge.

Related on excodex

• Ignition sources in hazardous areas: the 13 ignition sources and how to control them
• Earthing, bonding and equipotential bonding in hazardous areas
• Which ignition sources are most often overlooked in hazardous areas?
• What bonding resistance is needed to control static electricity in hazardous areas?
• Standards space: EN 1127-1

Standards and further reading

• TRGS 723 (Germany), section 5.6 on equalising currents and cathodic corrosion protection: bgn-branchenwissen.de
• A justification into the use of insulation flanges (and electrically discontinuous hoses) at the ship/shore and ship/ship interface, paper hosted by SIGTTO, summarising ISGOTT: sigtto.org (PDF)
• EN 1127-1:2019, explosion prevention and protection, basic concepts (ignition source "stray electric currents, cathodic corrosion protection"): sample pages
• ISGOTT, International Safety Guide for Oil Tankers and Terminals (ship and shore electrical isolation).

This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.