Multiple Cables Per Phase

How are the cables run and terminated? If run in groups of all the same phase you will get problems.

One of the best ways is to put one of each phase in trefoil formation, each trefoil group spaced apart.

Something else to consider is magnetic repulsion during a fault. A guy in the UK was killed when the unsecured cores suddenly flew apart causing a trench cover plate to fly in to the air. I used aluminium trefoil cleats.

This was a test scenario, but they were generally lying on the floor, not clipped in . Each phase would be grouped to allow current monitoring and connection to the star point

I understand the issues during short circuit, but for now I'm more interested to understand if the issues seen were due to the low voltage of the test.

Thanks

Yes, it is due to the extremely low voltage, but maybe it was used to make the test more stringent, and reveal any unbalance of the contacts closure.

The contacts of the ACB are still being that: contacts, subject to oxidation and contamination; You surely have worked on contactors or relays which (closed)contact's resistance range from a couple ohms to several hundreds, but they work perfect in the actual application.

Anyway, you stick to whatever codes apply for testing and/or installing such devices.

Regards

Hi Ramconsult Sounds like a good place to start. Perhaps one may give some thought to the possibilities of eddy currents especially in the cable glands and gland plates in the panel.

IMO the current unbalance could be in both situations : unbalance due to a wrong laying sequence of cables-mainly if the cables of the same phase being grouped together- and could be the low level of voltage [in this case a insensible difference of a few volts could be a big relative voltage unbalance.

According to:

A CABLE CONFIGURATION TECHNIQUE FOR THE BALANCE OF CURRENT DISTRIBUTION IN PARALLEL CABLES by San-Yi Lee [Fig.1] the current unbalance could be up to 1/7.

I assume you were pumping fairly hefty levels of current through the cables and breaker, in order for them to get hot.

Since you have that many cables in parallel per phase, differing resistances in each cable will affect how much current each will carry. Before the temp rise test, you need to check each cable end-to-end (including the bolted connections and the terminal lugs) with a low-resistance ohmmeter. Resistance should be in the hundreds of microohms range or less, I would expect.

If not, then there is some element in the cables that is causing higher resistance. It may be the bolted connection. Maybe the terminal of the breaker or the lug on the cable doesn't have a nice, clean, non-oxidized, flat surface. Maybe the lug wasn't crimped or bolted tight onto the cable end. There are many possible contributing factors, but the likely overall cause is higher resistance in some of the cables than others.

These effects will show up no matter what voltage. P = I²R. You will generate watts based on the current and resistance, with higher resistance cables causing more of a voltage drop across them.

Sounds like the resistance of the connections is much higher than the cables. Make sure the connections are clean and tight.

The mutual induction between parallel conductors may provoke an unbalance between currents flowing through these conductors. If the conductor positions are symmetric-as in an equilateral triangle-then the induced EMF from the other conductors is the same in all conductors and in turn all the currents are the same. If the conductors of the same phase are arranged in straight line-with clearing space or not-the unbalance is elevated-more conductors per phase, more unbalance.

In the above case of 7 cables per phase grouped per phase in a straight line the unbalance is 1/7[80/600 A].

The low voltage is important. It causes the best heating with the same power applied, and fastest results. I only have seen it done from a distance, smarter people on this forum know much more than me about this.

BTW, why not join this forum. It seems like you have something to say, and joining would distinquish you from the hundreds of noobs, and exam boys out there.

One can use conductor material is made by paralleling phase conductors then by using the large conductors for phase, yeah most of these huge imbalance is an effect of the low voltage, multiple cables per phase with proper modeling is good for the circuit.

Dear Friend,

The problem is in the way the Cable is connected.

You have stated "Single ACB in an enclosure acting as a main intake switch from a generator, each phase fed with 8x240mm2 cables, both in and out. Cables all the same length"

This type of connection gives a message that - though the cables are identical, 8 cables are CONNECED to R Phase, 8 cables are CONNECED to Y Phase, 8 cables are CONNECED to B Phase. If so this is wrong since the capacitive effect is compounding and results in HEATING EFFECT in the cable.

The correct method of connection in the given scenario is

"The RED Core of all the 8 cables to be grouped and connected to R Phase, the YELLOW Core of all the 8 cables to be grouped and connected to Y Phase, the BLUE Core of all the 8 cables to be grouped and connected to B Phase" This will nullify the EXCESS HEATING EFFECT of the Cable due to CUMULATIVE EFFECT of CAPACITANCE is NULLIFIED.

The reason for the unequal current may be due to some loose connection, improper crimping of Lug(s) adding to Resistance and resulting in over heating.

DHAYANANDHAN.S