Earthing of cable tray body
Earthing of cable tray body
(OP)
Why we put yellow/green earthing connector between parts of cable trays? does this have any relation to the bolts&nuts connecting the parts of the cable tray? (they cause a hotspot if any leakage current passes for example? or what?)






RE: Earthing of cable tray body
mechanical connection of two parts sometimes is not an adequate electrical connection. So it's better not to mix them up.
RE: Earthing of cable tray body
See NEC 2014 Art.392.60 Grounding and Bonding.
RE: Earthing of cable tray body
RE: Earthing of cable tray body
In North America the ruling code is the API (American Petroleum Institute) which is in addition to the Canadian Electrical Code (CEC) or National Electrical Code (NEC).
Under CEC or NEC codes, equipment fastened to a grounded structure is deemed to be grounded and no additional grounding is needed.
Not good enough for plants processing explosive and flammable products.
Under NEC and CEC codes, there may be a significant voltage on the surface of equipment in the instant before the protection trips.
Example:
A motor is cable fed from a solidly grounded 277/480 Volt source.
The motor is grounded by the grounding conductor in the supply cable.
There is a fault to ground in the motor.
The path of the fault current is from the source, through the supply conductor to the fault and returning through the grounding conductor in the supply cable to the source.
The impedance of the supply conductor and the impedance of the grounding conductor form a voltage divider across the 277 Volts to ground.
When the supply conductor and the grounding conductor have equal impedance the voltage at the fault location and on the surface of the motor may be expected to be 1/2 of 277 Volts or 138.5 Volts.
In practice the cable grounding conductor is often 2 gauge sizes smaller than the supply conductor and so has a slightly higher impedance.
As a result, we may expect a ground fault in a motor to develop a momentary voltage to ground on the motor surface of over 138 Volts.
This is unacceptable in an environment where flammable and explosive liquids and vapors may be present.
To avoid the possibility of momentary high surface voltages on electrical equipment, grounding to API standards in a petro-chemical plant is much more stringent than basic electrical code requirements.
Bill
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"Why not the best?"
Jimmy Carter
RE: Earthing of cable tray body
RE: Earthing of cable tray body
RE: Earthing of cable tray body
RE: Earthing of cable tray body
RE: Earthing of cable tray body
RE: Earthing of cable tray body
Thank you Scotty, thats a good point. Can you just clarify the second part "I wouldn't rely on containment as part of my designed earth fault path to meet disconnect times.." for my understanding?
RE: Earthing of cable tray body
Dave
RE: Earthing of cable tray body
What I meant was that wherever possible I would always have a designed earth return path to the source via an earth conductor or cable armour, rather than rely on fortuitous paths through parallel paths, bonding, steel structures, mass of earth, etc. Tray bonds are generally too small, too prone to corrosion, too prone being snapped by scaffolders and cable gangs, etc to be a reliable earth path in the long term.
In many cases the earth loop impedance dominates the fault disconnection time rather than the phase-phase short-circuit, so minimising the impedance of the earth return path is an important aspect of design.
RE: Earthing of cable tray body
RE: Earthing of cable tray body
Assuming there are no other specific [client, or legislation etc] requirements, and you accept the joint in lieu of a bond, you would need to have confidence that the joint would be treated with the required "respect" throughout its life [construction, and operation].
I propose a bond is immediately recognised.
Regards,
Lyle