trefoil cable and a steel frame transit.

[SilverfoxUK]

All

I'm running 3 phases and a neutral as separate cores in trefoil from a transformer.
The cores separate when I get to the metal frame transit. All phases run through the same aperture.
Do I need to apply a saw cut to prevent induction heating at the transit frame?

 

[davidbeach]

"...cores separate..." "...run through the same aperture..." Please clarify.

If there is a ferrous loop around anything less than all three phases there will be circulating current in the loop. If all three are enclosed in the same loop there is no issue.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[SilverfoxUK]

transit has 2 x sections or loops, I've got the 3 x phase cables and neutral through the same section.
I've gone for non ferrous gland plates

 

[che12345]

SilverfoxUK (Electrical)(OP)1 Nov 23 16:08
"...I'm running 3 phases and a neutral as separate cores in trefoil from a transformer. The cores separate when I get to the metal frame transit. All phases run through the same aperture. Do I need to apply a saw cut to prevent induction heating at the transit frame? ....#2. transit has 2 x sections or loops, I've got the 3 x phase cables and neutral through the same section. I've gone for non ferrous gland plates. "
1. If the (3C cable + the separate Neutral core) go through the metallic frame transit, no saw-cut to prevent induction heating is requited.
2. My understanding is when arriving at the gland-plate, the 3C cable separate into A, B, C . Therefore, there will be four cut-outs/holes; each hole for A, B, C and N respectively. I've gone? for non ferrous gland plates. If the gland-plate is of non-ferrous material ( e.g. brass, aluminium, fibre-grass etc.) you do NOT require to cut the holes into half.
Che Kuan Yau (Singapore)

 

[SilverfoxUK]

thanks for sharing, some good advice.

 

[waross]

Are your cables shielded or armoured?
You may want to consider an insulating gland plate at the load end to interrupt current due to induced voltages in the armour.
Gland fittings should be of non-ferrous material.
The locknuts on the gland fittings should be non-ferrous.
I once passed this advice on to a crew installing multiple single core cables on another site.
They didn't believe me and used steel locknuts. (They did use non-ferrous connectors but with steel locknuts.)
The AHJ demanded that the locknuts be changed to non-ferrous.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

SilverfoxUK (Electrical)(OP)1 Nov 23 16:08
"....I'm running 3 phases and a neutral as separate cores in trefoil from a transformer....."
1. I take it that: Cable #1, a 3C armourded cable,
Cable #2 a 1C a) armoured or b) non-armourded,
Conductor #3. a PE bare conductor.
2. Now look at how to earth the Cable #1., leave treatment of cable #2 separately.
2.1 The armour shall be earthed. At both ends preferred; with some current de-rating. Earth only on one end, either the sending (up-stream) or the load end (down-stream) is debatable.
3. If earthing at the up-stream is chosen, the down-stream armour is isolated from earth. This can be done with the armour stop and taped up say 30mm away from the gland.
3.1 Attention: do NOT terminate the armour on an isolated conductive gland plate. Reason being it is difficult to maintain the isolation/insulation to earth. An alternative is to terminate the armour on an addition plastic/non-conductive box. This plastic box is bolted on the metallic load enclosure etc...
Che Kuan Yau (Singapore)

 

[waross]

If earthing at the up-stream is chosen, the down-stream armour is isolated from earth. This can be done with the armour stop and taped up say 30mm away from the gland.

Grounding at the supply end is one standard practice under the Canadian code.
In the event that connecting the armours together by grounding or some other means creats objectionable circulating currents, insulating the armours at the load end is one accepted mitigation practice.
Other acceptable means are;
1. De-rating the cable ampacity to allow for the added heating of the cable by the circulating currents.
2. Transposing the armours at 1/3 points or multiples there-of so that the induced voltages cancel.
Note: When the armour is insulated at the load end, a separate grounding conductor must be used to ground field equipment.

Personal note to Che Kuan Yau.
Dear sir. I have called you Mr. Che as an indication of respect.
I spent many years in Latin America and in that culture Mr. Che would be appropriate.
While I don't always agree with you I do respect your technical knowledge.
Our disagreements are mostly based on different codes and different local practices.
More interesting than serious.
If you find my use of Mr Che to be inappropriate, please inform me of your preference.
Thank you.
Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Dear Mr. waross
I am a male septuagenarian coming to be octogenarian by next month. A Singaporean Chinese. My surname is Che; therefore Mr. Che is the "usual/accepted" way of addressing.
Thank you, Sir.
Che Kuan Yau (Singapore)