Ground cables for Star-Delta starters to Motors

[NickParker]

When we run parallel runs of cables for star-delta starters, (3 phase, 6 wire cables); Say, If I run 2 sets of 3 core cables, is it necessary to run 2 runs of separate ground cables too.

 

[Sazirul]

I am also looking for the right answer. But at my place, we used 1 run of ground cable for reducing cable cost. I know its funny, but my senior did that.



Fuses | Electrical Isolator | Busbar VS Cables

 

[waross]

Under the Canadian Electrical Code each set of cables must have a grounding conductor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

Are these cables not armoured! If these are armoured, armour itself can be utilized as earth continuity conductor (It can be earthed at both ends as it is a 3-core cable).
Laying one earth continuity conductor per trench and bonding it to all the steel cable trays in the trench is done to ensure all the sections of steel tray are bonded to earth.
Laying separate ground cable with each power cable seems too much.

 

[waross]

It depends on the type of armour. Using a spiral armour as a grounding conductor is a very bad idea.
Over time the armour tends to lose continuity between turns and becomes a spiral, inductive heating element.
"Laying separate ground cable with each power cable seems too much."
Argue that with the Committee on Canadian Electrical code, Part I

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr NickParker

Q1. Say, If I run 2 sets of 3 core cables, is it necessary to run 2 runs of separate ground cables too.
A1. It is permissible to run 2 sets of 3-core armoured or non-armoured cables [depending on the mechanical damage protection requirement]; from the starter to the motor. It is also permissible to run 6 single-core non-armoured wires. For earthing conductor, See below A1.3.
A1.1. The earthing conductor can be either of [copper or aluminium], which may not be the same material as the power cables.
attention: Use of aluminium earthing conductor would end up with a larger conductor size (in mm2) and may involve with connection problems etc.
A1.2. The most important consideration is the (cu or al) conductor [must] be adequately sized (in mm2). The other important consideration is the mechanical damage protection.
A1.3. The earthing wire may be [a [[separate]] single-core wire or multiple of single-core wires] either insulated (green/yellow in colour to IEC , green to NEC) or non-insulated. It can be a [separate conductor or conductors] which has nothing to do with the type or number/material of power cables.
Che Kuan Yau (Singapore)

 

[dpc]

If you're running them in two separate conduits, you need a grounding conductor in each conduit. If they are in the same conduit or tray, you only need one.

 

[che12345]

Dear Mr dpc

"If you're running them in two separate conduits, you need a grounding conductor in each conduit. If they are in the same conduit or tray, you only need one."
1. I do not recall in IEC or NEC specially stated so.
2. I am of the opinion that:
a) It is a good practice to run the earthing conductor [together] with the power cable. However, it is permissible to run two separate conduits, one with an adequately sized earthing conductor and the other conduit [without] an earthing conductor. Or, each conduit with [half the required size] conductor.
b) It is also permissible to run a single-core or multiple singe-core insulated or non insulated earthing wire in another separate conduit [without] any power cable; or on a cable tray.
Attention: It is [not] permissible to run [only one single-core [[power wire]] ] in a metallic conduit.
c) It is also permissible to run the earthing wire taking [a different route] back to the earthing bar in the MCC, where the starter is located,
e.g.the earthing wire is connected to the footing/frame of the motor instead of the [earthing terminal] located [inside] the cable box.

Note: earthing of the [conduit] is a separate matter, not covered in this discussion.
Che Kuan Yau (Singapaore)

 

[waross]

This is an international forum and what is acceptable in one jurisdiction may not be acceptable in another jurisdiction.
Some effects may have been identified in one jurisdiction, but not recognized in another jurisdiction.
My friend, dpc, works under the NEC which is very similar to the Canadian Electrical Code.
I worked for over a decade in an area where there was no electrical code.
It's surprising what is permissible and what works, sort of, if you are not bound by code restrictions.
That is if you are not bothered by a few minor annoyances:
Conduits too hot to touch comfortably.
Paint burned off of enclosures.
Enclosures too hot to comfortably touch.
A 400 Amp rated switch repeatedly destroyed by a 200 Amp load and heat from improperly arranged feeders.
Wide spread burnouts of refrigerators over at least two distribution systems.
Some effects are more subtle and only arise under fault conditions. eg:
Improper routing of a grounding conductor may result in a grounding conductor circuit showing higher than expected impedance under fault conditions, resulting in slower than expected tripping of protection devices.
This in turn may lead to inaccuracies in arc flash studies.

However, it is permissible to run two separate conduits, one with an adequately sized earthing conductor and the other conduit [without] an earthing conductor.

Not under Canadian codes.
There is a reason.
You must run a grounding conductor in each conduit, however the grounding conductors are sized based on the cable size, not on the size of the feed breaker.

b) It is also permissible to run a single-core or multiple singe-core insulated or non insulated earthing wire in another separate conduit [without] any power cable;

Not where I live. This practice is absolutely prohibited by the Canadian code. There is a reason.

Attention: It is [not] permissible to run [only one single-core [[power wire]] ] in a metallic conduit.

This is permissible under the Canadian code if the current is below 200 Amps. This applies to ferrous conduits.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Beyond the code questions, you need to consider the purpose of a grounding conductor. In the event of a phase to ground fault, the grounding conductor provides a low impedance return current path for the fault current to allow enough fault current to trip the upstream breaker or protective device. If you are relying on a ground conductor that is not in the same conduit as the phase conductors, the inductive reactance of the circuit increases drastically and many times there is insufficient fault current to clear the fault. This is dangerous.

Electrical codes are not design guides and what is or is not allowed in a particular code does not change the laws of physics. A grounding conductor is needed in each conduit regardless of what any code might allow.

Cheers,

Dave