View Full Version : When it went bang it made me jump!

12-11-2012, 10:16 PM
OK Guys, are you sitting comfortably?

I got asked by a good customer to attend his site last week, as his local controls specialist (knows his electronics).
Could not identify why at various run times, his number 2 of 3 screw Comps was tripping its 73a motor overload.
All the usual questions like run pressures and temps were answered correctly particularly the HP side.
Basically as was explained to me, the errant compressor would run for anything between 1 to 6 hours prior to it tripping its Delta contactor overload (set at 73 Amps).
Apparently no-one had been present when the plant tripped.

Off I trundle 150 miles one way.
On arrival the plant is shut down and at my disposal. So first things first, all the contacts and terminals from the Star Delta contactors through to the Motor terminals (40 kw motor).
Are metered for resistance, visually checked for corrosion etc and windings meggared.

Nothing un-toward came to light.

OK, the comp along with the other 2 was set to work. All three were pulling similar amperages across the 6 windings.
As the overload in question was on the Line contactor, I monitored the line phases with a clamp meter.
All 3 comps showed the middle white phase as the highest, with No 2 comp pulling the most.

As the load increased so did the white 2 phase. I actually watched this increase to 72 amps once momentarily. Although its average was 68 /69 amps, to high for my liking.
At which point it dawned on me that I had never witnessed the motor bearing ever having been greased.
A quick conversation with the site engineer confirmed that they had never been greased to his knowledge in their 21 years history.
Stal R50 mini screws for those that are interested, built to last is all I can say!

The Engineer returns with some suitable lithium grease which is used to grease the motors.
Whilst him and I are doing this there is a almighty clunk clunk from the comps electrical panel.

I shoot round to it's front staying there when all of a sudden the No2 comp delta contactor starts dropping in and out in unison
with the star delta timer.
These are 90amp contactors mind! It ain't pretty and more than a little scary when contacts
of that size start chattering.
I quickly turned the no2 comp run switch off and watched with the engineer who had by now joined me at the panel, the delta contactor continue to drop in and out.

So as there was a replacement star delta timer on site it was changed and the greasing of the motors was completed.
On start-up the average motor phase amperage had dropped by around 10 amps.

The plant has not (I believe!) had any further issues since.
Try finding that one in a manual, hence my post.

Oh! and the irony is my friend and colleague whilst discussing the issues earlier that morning has said " I wonder if there is an issue with the star delta timer".
I phoned him up later and accused him of being psychic.

I did take credit for the greasing of the bearings however!


install monkey
12-11-2012, 10:56 PM
I had a carrier 30hr as an apprentice, found the 75amp breaker tripped and 1 part wind contactor welded, so I replaced both for klockner 63amp, all sorted , switched the bugger on and after the step controller motored round BOOM- pot was dte, took out the 200amp fuses-4ft away from where I was standing, ears were ringing for about an hour!!!!!!- the joys of fault finding!!

13-11-2012, 03:13 AM
Hi Grizzly, oh yes terrifying stuff.
Had a super large centrif motor fail between phases at full load/ 400 amps. Two incomer HRC fuses the size of tree trunks exploded, 3 metres away. similar noise to a howitzer being fired, definitely skid marks in the undies that day.

13-11-2012, 10:04 PM
Had the pleasure of being roped into a HV DC ACB test one day .
For those not involved in the "Multi Skilled" Facilities game it involves dropping out HV mains voltage under load to simulate a severe mains failure akin to a lightning strike.The normal procedure is off line switch to 590v DC for about 3-4 mins to allow for generator sync (about 15mw) then through ACB for rectifier sync at about 1500 amp for this particular installation .
The day previous external contractors were involved in a PPM inspection and tagged this particular ACB "good for test"
So we plugged on with the test...
The ACB they had actually PPM'd was the one beside it ,and whilst removing it they had missed place a washer, which had rolled across the flash guard and dropped behind the one we were switching.
It did not end well....
Myself and the electrician ended up in hospital ,burst ear drums and a dislocated shoulder was as bad as it got for me ,but the sparks who was standing in closest to it when it went off suffered 3rd degree burns to his arms and chest 8 broken ribs and a shattered pelvis not to mention the shrapnel .
We both made a few quid out of it and were lucky enough to be around to spend it.....But it's a fine line.....So as the saying goes ,never trust a professional, check it yourself!!!

PS.Kudos to your mate Grizzly,good shout!!

14-11-2012, 01:30 PM
I had a carrier 30hr as an apprentice,

probably better than a real one

Rob White
14-11-2012, 02:21 PM

To my shame when I was a lot younger I had a brain fa*t and
while testing a large 3 phase compressor, I pushed the third (star)
contactor in................

If you know anything about electrics you will know what happened next.

An almighty flash and the loudest bang I ever want to experience.

Burns up my arm and 3 200amp fuses blown. The contactor disintegrated
and I was very, very lucky not to have been seriously injured.

I was so humiliated when I had to ask for help from another engineer,
to put right what I had so catastrophically put wrong.............

I won't do that again in a hurry.



14-11-2012, 04:29 PM
I had a carrier 30hr as an apprentice, found the 75amp breaker tripped and 1 part wind contactor welded, so I replaced both for klockner 63amp, all sorted , switched the bugger on and after the step controller motored round BOOM- pot was dte, took out the 200amp fuses-4ft away from where I was standing, ears were ringing for about an hour!!!!!!- the joys of fault finding!!
Tutut should've megged the comp first :p

14-11-2012, 11:03 PM
Add the above to some of the horror stories I have received off the forum.
Then I begin to realise that we "Fridgies".
Deal with a wide spectrum of kit, that can include some serious electrics. All of which is not talked about enough.

So next time the younger engineer thinks about questioning why use a proving unit or "dry" test circuits.
You can tell him read the forum!

Reading actual experiences from colleagues on here is much better than being told by some H&S (Well Meaning trainer) who has downloaded info from the Internet.
Thanks for allowing us to learn from your mistakes guys!


15-11-2012, 08:55 PM
So next time the younger engineer thinks about questioning why use a proving unit or "dry" test circuits.
You can tell him read the forum!

Thanks for allowing us to learn from your mistakes guys!

I am the 'younger engineer' and this type of information is what makes this forum.

I thank you for bringing this subject up, very interesting reading. Hearing of your real life experiences is not the stuff you learn from books.

I remember visiting a site where a couple of site engineers had been injured when working on electrics. They both suffered burns.

It gives you great respect for what you are dealing with.



15-11-2012, 10:20 PM
If it's any consolation Charlie, we all never stop learning!
I totally agree with your sentiments though

Just some of us forget more nowadays!
So I enjoy the reminders now and then.
Where else can anyone access such a wealth of knowledge?
God bless the INTERNET.

Rob White
16-11-2012, 08:30 AM

At work one day I get a call from the office.

"Rob get round to Marks a Spencer now"

Why I ask

"just drop what you are doing and get there"
There has been an accident and you need to put it right.

"OK" says I and heads straight off.

I'm there in minuets and as I park up there is a ambulance
in the back yard.

Duty manager takes me aside and explains the situation.

Experienced electrician while working in the electrical switch
room shorts out the main bussbar and showers himself with
molten copper, but in the flash and bang he was thrown backwards
and knocked himself out on a wall behind, also suspected broken
ribs :eek:

I have to repair and get power back on ASAP because the fault
was between the emergency back up generator and the mains
supply, result was no power to the entire shop.




Rob White
16-11-2012, 08:49 AM

Have you ever heard of the saying that if somebody is useless at their job they are promoted out of the way!

When I was working in a large pet food factory as the resident engineer looking after the refrigeration
equipment we had a supervisor who was a fitter by trade and although a nice bloke, he was not that good!

One day one of the other engineers was doing a cylinder liner replacement, so he had the heads off
the compressor and was slowly rotating the crank by hand to ensure that each piston was moving freely.

Two mistakes were made that morning............

One was the lad doing the work did not lock off the compressor motor and the other mistake was, they let
the supervisor into the engine room

"why are you rotating that by hand he states" and proceeded to press the start button................

Imagine the chaos........ The comp does one complete rotation and throws out every liner,
it then attempts a second rotation and each unsupported piston falls over in the body of the comp
on the third rotation it jams the pistons into corners and bends the con rods.

Before he even got his hand off the button the supervisor damaged thousands of 's worth
of comp and another 3 or 4 days to rebuild......

Ps I was not involved in that one fortunately :cool: .



16-11-2012, 02:28 PM
I remember one day after I did a Trane CentraVac CVGE 45 compressor rebuild. All went well till the day of commissioning. Rigged compressor in, evacuated system, break vacuum, did stand test. Started chiller, all started well as I walk around to oil tank I heard a loud Bang!!!!!!!. When I checked I found the panel door which was locked ripped opened. Was transition resistors, transition contractor got stuck in during star delta transition. I never had that experience on Trane chillers. It was terrible.

16-11-2012, 10:04 PM
Yep, all the schooling in the world wont teach some of these faults.

A fomer collegue told me when he was asked to change some fan motors on a Waitrose pack one saturday.
As usual nothing marked so each time he turned the main isolator for the pack off and changed the 3 phase motor. Each one went well until the last one which was spinning the wrong way. Right oh he thought, isolator off, swap the wires on the contactor, isolator on, BANG, sound of whole store gradually winding down with no power.
Manager appeared doing his nut, tech wonders what went on. After all he only swapped 2 wires on the contactor.

Any takers yet........

An electrician was called to sort out the power out meaning the store was without power for the rest of the afternoon and into the early evening. the store was a ghost town with abandoned trolleys all over the place.
The bang came from one of the main incoming fuses blowing (baked bean tin size version). This was caused by the pack isolator breaking up inside so when it was turned on for the last time 2 phases ended up connected.

That story is always in the back of my mind each time I turn an isolator on now

17-11-2012, 03:24 PM
Have had a couple fall to pieces, a few cremate themselves but never any go phase to phase short without symptoms.

12-02-2013, 09:31 PM
We had an "electrician" in helping us with the wiring involved with installing three screw compressors. I started to have suspicions when he called me over a couple of times to ask me what color a particular wire was. He was color-blind and they all looked gray to him. Anyhow, He had finished wiring up the starter of a 250 HP motor and apparently didn't like the way it was pulling in the contactors.

I was doing something across the room and happened to look over again just in time to see him pushing something in the panel with a long 2x4. He broke and defeated the mechanical interlock between two contactors and shorted out all three phases.

Fireball. Luckily he was back far enough to avoid anything serious.