Cable Tray

[Redskinsdb21]

Hello folks,

Just have a general questions. Its in regards to grounding cable tray. If you have a long run of cable tray that is properly grounded. I understand that grounding is in case a cable that's on the cable tray becomes shorted it will allow current to flow to ground until breaker feeding cable trips. Or at least this is how I think it works. My question is how is it safe for folks to work up in pipe racks around the cable tray. Is this not dangerous dur to the possibility of a cable in the cable tray being shorted and thus causing electrocution? Just a thought while visiting a plant and seeing folks work near cable tray with 4160 V energized cables in the tray.

 

[Sn00ze]

I am not a grounding guy but I figured i'd weigh in.

It 100% is dangerous. That is why only qualified personnel should be allowed near it even when everything is working properly. Bonding and grounding minimizes this hazard by giving the short/fault a path to ground, instead of it being the person touching the cable tray. That's the whole point. It will still hurt but it will prevent death or serious injury.

The other thing to consider is the construction of the cable. a lot of those industrial cables come with a shield or a ground built in to prevent this.

 

[che12345]

Dear Mr. Redskinsdb21 (Mining)(OP)13 Jun 22 18:19
"... #1. Its in regards to grounding cable tray. If you have a long run of cable tray that is properly grounded. I understand that grounding is in case ..... it will allow current to flow to ground until breaker feeding cable trips.... #2. My question is how is it safe for folks to work up in pipe racks around the cable tray. #3. Is this not dangerous dur to the possibility of a cable in the cable tray being shorted and thus causing electrocution? Just a thought ....seeing folks work near cable tray with 4160 V energized cables in the tray "..
!1. Yes. This principle (grounding/earthing) is being the practice by IEC/ANSI/NEC on LV/MV/HV. See NEC 250-xx.
#2. (per NEC 250-86 ) with the (metallic) cable tray [properly grounded/(effectively grounded per NEC) ; the potential between the tray and ground will be minimized, which reducing the shock hazard to personnel...
#3. See #2. above. It is 100% save. It is usual practice to lay MV cable on a separate tray from that of the LV cables. The trays shall be bonded. There is no requirement [to separate (for any clearance distance)] or [labelling the cable trays of different voltages]
Che Kuan Yau (Singapore)

 

[che12345]

@ Dear Mr. Sn00ze (Electrical)13 Jun 22 19:02
".... #1. It 100% is dangerous. #2. That is why only qualified personnel should be allowed near it even when everything is working properly. #3.Bonding and grounding minimizes this hazard by giving the short/fault a path to ground, instead of it being the person touching the cable tray..... It will still hurt but it will prevent death or serious injury ".
#0. Corrigendum. My earlier post dated 14 June 22. "...#3. See #2. above. It is 100% save" , should be " ....It is 100% safe.
#1. I am of the opinion that it is 100% safe.
#2. No. There is NO restriction to any person/layman to come into contact/touch an effectively grounded cable tray with MV cables inside it. A MV cable tray is allowed to run side-by-side with the separate LV cable tray. The two separate trays shall be bonded, but NO separation distance/clearance need to be observed.
#3. Any MV/LV cable tray which are effectively grounded would maintain/ensure that the potential to a safe level; that do not hurt or cause any hazard/injury.
Che Kuan Yau (Singapore)

 

[JezNZ]

1. Cable Tray is not really required to be bonded if carrying sheathed cable and there is no mechanical hazard and it is mounted above a 2.5m height. (Unless you have a separate legislative requirement)

2. The bonding conductors cause a low impedance earth return path. This primarily for the purpose of the operation of the protection in a short period of time so the cable ladder is not permanently live in the case of a fault where the MV cable screen fails.

3. If this is an industrial facility with impedance to earth <1ohm and all equipment is cross-bonded, at 4160V, the risk can possibly be negligible.

4. A spurious cable fault mid-cable is very rare (~10% of failures) and is more likely to result in an Arcing Fault due to impact damage or strike. These rarely cause direct electrocution but rather cause an Arc Flash/Blast which can cause significant burns. For this reason it is good practice to protect the cable ladder by enclosing with covers.

5. I agree completely with the assessment of Mr. che12345

 

[che12345]

@ Dear Mr. JezNZ (Electrical)16 Jun 22 06:08
"...#1. Cable Tray is not really required to be bonded if carrying sheathed cable and #2.there is no mechanical hazard and it is mounted above a 2.5m height. (Unless you have a separate legislative requirement)..."
I am of the opinion that:
#1. All a) MV (sheathed/without sheath MV cables) and b) those with LV cables trays shell be effectively grounded. At site, MV and LV cable trays may be installed at close proximity. There is every possibility that personnel may come into contact with MV and LV trays at the same time. " Bonding " them is to ensure the trays are maintained at the same potential.
# 2. The purpose of Bonding as explained above has nothing to do with "... mechanical hazard and it is mounted above a 2.5m height".
The principle/purpose of bonding is the same in IEC/ANSI/NEC.
Which national/ international legislation/regulation/standard are you referring to, please advise.
Che Kuan Yau (Singapore)