Over Heating of Cable Terminal

other single phase load

I smell an imbalance here. I suggest eliminating some of the loads and rechecking your amperages. Should be a simple matter to even things out if this proves to be the problem.

1 What is the ampacity of 150 MM AL. I could only find copper

2. What kind of connector are you using?

3. Harmonics?

1. Ampacity of 150 sqmm Alu. Cable is 205 Amp.

2. Aluminium Thimble suitable for 150 sqmm cable we are using.

3. How to check Harmonics????

Check harmonics with a spectrum analyzer.

→ British Standard 7671.

Hi,

The contact resistance of the Aluminum lugs need to be checked. What method you are using. Is it crimping, what tool and die is being used.

Secondly, as mentioned By Sridhar, in the event of contct is between two different metals, bimetallic /mechanical lugs can be used.

Thirdly, what is the surface area of the contact. What is the hole dia of the lug.

ou can contact Raychem rpg for complete solution.

Regards

Amit Bhatia

Are you using bi-metallic(aluminuim-copper) terminals? if not then you might want to change from copper or aluminum to aluminum-copper terminal, that causes overheating on cables.

150 mm² is almost equivalent to 300 kcmil. Per NEC, the ampacity of this size aluminum cable is nearly 200A (even with ambient temperature correction and 75° C temperature rise). So, in terms of ampacity, 150A current is not the overload to the cable.

In addition to pointing the finger to the termination, I am also concern about Op's statement 'the 250A breaker gets faulty due to excessive tripping'. With 150A current, the 250A breaker shouldn't trip. If the 250A trips, the current is almost close to 250A range. So, I suggest verifying the current again with precise clip-on meter.

The current may have harmonic components but the load you mentioned doesn't indicate that it may have such a level of harmonic components. However, based on the nature of the other single phase loads (if they are nonlinear nature), harmonics may be also be the cause of cable overheating and breaker trip.

- MS

it my sound silly i had this experience, while cable termination with Lugs was going on the workman had used undersized lugs carelessly and since he was unable to push the cable in to the lug he had shaved of cable strands and finished his work it took quite a long time to identify the fault and solved in an hours time i had 20 terminations to cope with in one circuit

crm

ex power user

it makes a big sense, this happens quite often if you don't supervise or trained your jointers.

Please check the installation.

Bad joints at the termination may be the problem.

Check Termination lugs.

Palm of the Terminal Lugs may not sit properly at the termination Tag.These should make Firm contact.I think this is In India, I have noticed in many places it used to make point contacts.

Crimping of the Terminal lugs may not be perfect.

I notice many joints in this circuit.

Check the installation patiently

The major issue is going to be with the AC loads. When the AC cycles on, you get high starting currents that you would not necessarily see when taking instantaneous current measurements. Starting current for the AC compressor can be as much as 10 times the running current, although it is most commonly closer to 5 times the starting current. You should do a time log of the current, looking for any spikes above your steady-state current. This could also be the cause of your frequent breaker tripping. It is also possible that your current measurement equipment does not have sufficiently rapid response to record the starting current spikes.

Burned connectors and tripping breakers are definitely an indication of too much current.

Mr Srivastava,

There have been many good suggestions so far, and i hope you are taking them all seriously and checking out your installation. You will surely find the answer.

One doubt here .. you have a 250A MCCB at the start, and a 400A MCCB at the end ? Should have been the other way ?

i suspect improper termination and inadequate tightening torque where the cables are terminated. We are talking here of burning of metals.... requires a high temperature. Maybe the frequent tripping of the MCCB is not due to overcurrent, but due to extra heat from the terminals if it is a thermal/magnetic MCCB.

i have even seen micro-arcs at loose terminals. The temperature then is naturally enough to burn without tripping any protective device. You may see such arcs at badly done distribution transformer wire joints on a dark night.

kvsridhar has a point there.

I had some problems with oxidized Al-Cu connections. Did you try to use infrared camera to check hot spots at the connections? if not better try it. It will tell you how hot the connections are.

As for the breaker, you should choose it according to the maximum short circuit current. Besides the maximum load, for rated current also take into consideration the starting current for motors an inrush current for transformers, if you have any.

yes Iagree with guru

Apparently your current and MCCB size are in order. But I think you have to redo the cable sizing exercise applying the following factors.

1-Determine the carrying capacity according to the type of cable insulation and method of laying i.e PVC insulated/ xlpe insulated and dirctly burried into the ground or laid in a duct or in free air.

2- Derating according to the soil resistivity.

3-Derating according to the ambient temperature.

4- Derating of there is grouping.

5- Calculate the voltage drop (check both voltage at the start and at the end). Don't assume that the feeding voltage is as declared by the power supply company.

Because normally the major cause of overheating is overloading. If the MCCB is adjustible seek close excess protection by adjusting its tripping current, but ofcourse after finalizing your calculations. I hope it will solve the problem. I think it is not to teach to an electrical professional to check that all the connections are properly tightened.

Just to stimulate the conversation,

could this be one of those things the magazines and salesmen talk about. For quite some time the notion of 'dirty power' has been hot.

The cause is usually said to be variable frequency drives and solenoid valves.

Not much is said about solutions, but you can buy a very expensive power analyzer meter or pay someone to come in and so it for you.

In my experience we just fix the damage and keep on turning out widgets.

Dear Shrivastavaji,

I think you are up against twin devils of electrical engineers- under size cable selection and poor workmanship on cable termination. I have a hunch that while selecting the cable for type of load, all the derating factors have not been taken into consideration. I am sure cable route is quite congested. The cable is not only getting heated up its own inductive load (predominantly AC) but by the surrounding cables as well. Considering all the required derating factors, the cable loading is just neck to neck. Now add to it starting currents of ACs each having its own starting cycle will keep the cable fully loaded all the time. And to top it all, poor termination has further accentuated the problem. Needless to say, Al cables are notorious for loose connections. You can, in fact, never be satisfied about its tightness. The other point I suggest is to use the two breakers the other way round. Rather, I would request you to consider use of ACBs instead of MCCBs or switches for the said application.

Many valuable points have already been posted and I am sure you will definitely derive solution for your problem.

BB Raina

I strongly agree with Mr Raina's diagnostic,the flaw in selection. But I beg to differ with the suggestion of using ACBs. It is commercially not viable considering the price of the ACBs, he can cheaply replace the cable with the next size in less expenses rather than going for ACBs.

I respectfully and partially object to the statement "Poor workmanship on cable termination".

I have several bad experiences with electricians, but I have to stand up for them on this one.

To many times I have found terminals loose that I think I (me) tightened, or at least I think were tight in the past.

I don't know why, but I think something happens to cause screw or bolt terminals to loosen.

I beleive that there can be value in killing the mains and going through all terminals and insuring they are tight. One or two are invariable found to be loose every time.

i believe that terminal connections which are quite tight when new, will become loose over time due to (1) differential expansion of the metals involved and (2) creep of the soft metals like Al and Cu. Hence the recommendation of a Belleville washer aka Disc Spring .. here is a nice link : busbar+bolting+schedule+disc+springs

Especially when working with mixed materials (i.e., copper/aluminum), thermal effects can result in loosening screw terminals over time due to differential thermal expansion of the different materials.

Dear All,

For last three days we are continuos tracking the load and found that the load on cable raise upto 199 Amp many time.

Now we are planning to put 400 Amp MCCB at input point of cable and very soon we are going to put one more 150 sqmm cable. Also as suggested we are planning to put Bimetelic thimbles.