All overhead cranes built in the USA are now required to have an extra grounding conductor – a fourth rail on conductor bar.
In January, a new version of the National Electrical Code (NEC) revised the existing standard, which had said that metal wheels running on a metal track would be sufficient to complete a grounding circuit of exposed metal parts of cranes that are not intended to carry current.
Section 610.61 of the NEC now reads:
“All exposed non-current-carrying metal parts of cranes, monorail hoists, hoists, and accessories, including pendant controls, shall be metallically joined together into a continuous electrical conductor so that the entire crane or hoist will be grounded in accordance with Article 250. Moving parts, other than removable accessories, or attachments that have metal-to-metal bearing surfaces, shall be considered to be electrically connected to each other through bearing surfaces for grounding purposes. The trolley frame and bridge frame shall not be considered as electrically grounded through the bridge and trolley wheels and its respective tracks. A separate bonding conductor shall be provided.”
“Once the rails get dirty, any kind of neutral back to the main system has a return through ground,” said Tom Young, electrical engineer at DeShazo Crane Company. And that neutral could be a piece of metal hanging off the crane, or a person. “Now there is a dedicated path to ground through the conductor bar,” he said.
The change will only apply to cranes built since January, when the code took effect. Older cranes will be exempt. “In most cases, the latest edition of the NEC is not applied to an existing installation unless the original installation was performed in such a manner as to constitute a demonstrable compromise in fire and shock protection for persons and property,” said a spokesman for the National Fire Protection Association, which publishes
the NEC.
Tom Young is also engineering chairman of the Crane Manufacturers Association of America (CMAA), which submitted a request for the change in 2002. It takes three years for the NEC to process updates.
According to Young, the fourth rail proposal was the first time the CMAA has proposed a change to the law. “Probably there are quite a few groups out there that don’t do it. This just gives it more teeth,” he said.
The CMAA has recommended the use of a fourth conductor bar for years. It recommends their use in its overhead crane specifications – Number 70, for top-running trolleys, published in 2000, and Number 74, for under-hung trolleys. Both were revised in 2004.
One of the CMAA’s interests was improving performance of variable-frequency drives, according to Young. A fourth rail helps reduce drive faults by draining away excess current, he says. The fourth rail also protects those working on or with the crane from getting shocked. This was another reason the CMAA campaigned to change the law, Young said.
In Europe, a fourth conductor bar has been required since June 1999, in EN 60204, part 32, section 8.2.3, according to Karl Tillmanns of German conductor manufacturer Vahle.
Young inherited the CMAA role from George Lovey of Demag Cranes & Components Corporation (of the USA) in 2003. He said Lovey began the campaign for a fourth rail.
Young’s term is now winding down. He said he is now applying to join the 26-member technical committee that produces the NEC. The crane position – code panel 12 – is vacant following the retirement of Morris/P&H’s Bob Royce, he said. Royce was the one who had worked on the fourth rail application.
Young said he was not aware of any other CMAA proposals to the 2008 NEC.
