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Earthing of cable tray body 1
- Thread starter masalah
- Start date
In my opinion, one does not need to use grounding jumper if the cable tray sections are bolted and the maximum short-circuit current will not be more than 600 A for steel tray or 2000 A for aluminum tray.
See NEC 2014 Art.392.60 Grounding and Bonding.
See NEC 2014 Art.392.60 Grounding and Bonding.
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1
- #5
If you are using ground jumpers on cable tray, it suggests a petroleum related plant.
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
--------------------
"Why not the best?"
Jimmy Carter
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
--------------------
"Why not the best?"
Jimmy Carter
If it's green & yellow then it is probably European or European-influenced. In the UK it is quite common to see continuity between cable rack or ladder maintained using tinned copper braid rather than wire links. The bonding may be required by the code you are working to.
electricnewbie123
Electrical
I believe it comes down to achieving the necessary resistance of the earth fault loop as stipulated by your local codes. If it could be proved by testing that without the copper bonding connections, the necessary earth fault loop impedance could be achieved, then they are not necessary. It will result in appropriate operation of protection devices. However, I also believe it is done at some places for peace of mind, because it just doesn't look right without them!
Yep, we do it in all installs to prevent sparks from jumpimg across cable trays to adjacent metal structures should cable trays get induction voltage from cables in them. Mostly required on highly explosive atmospheres, IMO.ScottyUK said:
This type of installation is essential in case of non-linear loads (VSD) supply cables (depends of EMC level you want to achieve in your installation, cable tray metal cover etc). Moreover as a designer you can calculate with higher accuracy the total resistance of ground path. Of course, as ScottyUK wrote, equipotential protection is a serious issue to take it for granted.
electricnewbie123
Electrical
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?
When I design a cable tray system, I always use a grounded bare copper cable EGC (sized for the largest oc device). Mounted by cable tray grd support clamps on the bottom side flanges (either inside or outside). This is to allow for the joint splice plates to be able to move for expansion and some other movements. The grd also comes in handy for conduit drop groundings.
Dave
Dave
Hi electrichie,
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.
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.
electricnewbie123
Electrical
Thank you ScottyUK - good explanation...
PersonalProfile
Mechanical
The attached may be of interest.
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
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
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