Burning of PVC coating of Mettalic conduit carrying data cables

[ajayshah]

I have encountered following problem at one of our site. Unable to think of a possible reason. Request for your valuable inputs
Problem: Outer PVC sheath of metallic conduit carrying data/voice cable burnt.

Problem ocured at a site where the distribution of data/voice cables is thru a metallic (MS powder coated}duct as a trunk route.
Each work station is fed 4 nos of cat6 cables from data duct to the work station thru a GI flexible PVC coated conduit with brass glands at either end. On one end this conduit terminates in the data duct and at the other end the conduit terminates in the data box on the work station.(No contact between the gland and the metalic encloser on either side, as the outer cover of conduit is PVC). (One of the cables cater to voice, one cater to data and other two are spare)

One day suddenly outer PVC sheath for one of the locations burnt heavily.

The affected nodes were 'dropped' from the data switch.

 

[waross]

Is it possible that someone was welding in the area and the welding cable inadvertently used the flexible conduit as a return path? Possibly an equipment failure at a work station and the fault current followed the conduit in question.
Is the work station grounded and is the grounding system intact?
In North America, most glands for flexible cable have an inner insert to support and ground the metalic flex.
If the burning was the result of electrical heating you will expect to see the damage uniform over the length of the flexible conduit.
If the damage is not uniform from end to end, look for an external source of heat.
respectfully

 

[ajayshah]

1. There was no welding any where in the whole building as the office is occupied for last two years.
2. No failure of any power strip on the desk.
3. Workstations are not grounded and are placed on carpet.
4. The burning was not uniform through out the length.
5. The burning is definitely not due to any fault current as no breaker has tripped any where

Any co-relation to High Frequency (Radio Frequency) induction heating ?

 

[waross]

3. Workstations are not grounded and are placed on carpet.

Actually, this can lead to the a similar type of problem, however the burning would be uniform.

5. The burning is definitely not due to any fault current as no breaker has tripped any where

There are conditions where a lot of damage may be done without blowing a breaker. One example is with an ungrounded work station or consul when a hot wire contacts the frame. With a good ground connection the breaker will trip. If there is an "accidental" ground of high resistance the high resistance area will get hot, possibly red hot.

Any co-relation to High Frequency (Radio Frequency) induction heating ?

Possible but probably not. It takes a relatively high amount of heat energy to do the type of damage you describe. Only a very small amount of the total RF energy radiated from a device is typically involved in incidents such as this. Most RF sources in an office radiate such low levels of power that they are completely incapable of doing any damage. Do you have any equipment nearby that radiates thousands of watts of RF power? Furthermore, enough stray RF to do such damage may also cause severe burns in nearby human bodies.
Have you considered chemical burning? A possible spill of a cleaning chemical.
respectfully

 

[waross]

I just came across an incident report that seems similar.
The cause was not definitely known but was supposed to have been a work light left in close proximity to the cables by a worker who may have quietly taken his lamp and left when he realized the damage he had caused.
Just a thought.
respectfully

 

[ajayshah]

The incident was below the false floor and in a secutarised area. The cause cited is not a possibiulity in this case. The occurance was aslo at more than one lacation. After the 'dropping' of the nodes by the switch the heating was not there.