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Abe
05-01-2006, 06:08 PM
If the current draw is 12.5 amps per phase on a 3 phase compressor, what should the overload protector on the contactor be set at ??

Ive set it at 14 amps...

Secondly,

The main circuit breaker on the Distribution Board, serving that compressor, is a Type C 32 amp .

Is that too big??

Over New Years, one phase was lost, shutting down all the refrigeration.

Fortunately no compressors were damaged.

I want to ensure that in future any such failure will promptly cut off all power to my systems.

I did not check on first examination what had tripped.

I know my 7.5HP Copeland scroll has an internal phase loss module built in.

But I have had a scare, thinking the worst. Situation was not helped when electrician came, switched phases, I told him specifically do not switch cables.

When I started up the scroll it was like a tractor was ploughing the field.

Also, electrician sorted out the lost phase, but didnt bother to check ancient isolator...........

One of the forks went handle pulled in On position did not make good contact

After he went I was still a phase down, plus the prob of him having switched phases....

Double trouble

In the end, I went all the way back, found sparky hadnt done his job properly, got him to sort, and finally Ive got all the systems humming along nicely.

For once, Im glad the outside temperatures are hovering around zero

Had this happened in summer....

Argus
05-01-2006, 06:52 PM
Aiyub, you have 3 points here.

1. The trip setting:
Generally a starter overload is set at 15% above the full load current, so 14 amps is about right, in my opinion.

2. The MCB size:
On the face of it a 32 Amp MCB is a little big for a 12 amp load, but that?s not the consideration or the answer.
The protective device?s main job is to protect the circuit, i.e. the cable; not necessarily the load on the circuit that should have its own intrinsic protection as an addition.
But we don?t know the cable rating. (not size ? rating). The rating of the supply cable to the unit needs to be greater than the rating of the protective device. So, is your cable?s actual rating less than 32 amps? If it is then the MCB is over-sized. If not it isn?t.
By the time all the rating factors are applied to a cable, temperature, clipping method, length, etc., its true rating will probably be considerably less than what you started with. But good practice will tell you to select a protective device as close as practicable to your load, whilst avoiding nuisance trips. 16 amps will probably do, if not 21 amps will be adequate. The MCB manufacturer will have full details and back-up.

3. The quality of the electrical work and the diligence of the technician:
That?s between you and your electrician! Due diligence would suggest to me that a basic visual check would be the minimum expected and alert you (and your client) to other problems.

.
________
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Brian_UK
05-01-2006, 07:48 PM
Also, if the sparky is liable to do work without your knowledge then perhaps you should build in a phase failure/rotation protection device.

frank
05-01-2006, 07:56 PM
The protective device’s main job is to protect the circuit, i.e. the cable; not necessarily the load on the circuit that should have its own intrinsic protection as an addition.


When I did the 16th Edition course (BS7671) a few years ago we where taught that the fuse (mcb) must be rated higher than the load and the cable must be rated higher than the fuse (mcb). This is all down to fault currents and disconnection times. From memory this is stated within BS7671.
Under fault conditions the cable must not be allowed to exceed 70C or it will require replacing. I'll dig out the chapter and try and post it later as the book is in the office.

We normally set the overload 20% above measured full load running current.

Abe
05-01-2006, 10:08 PM
Thank you both Argus and Frank.........

Now just one other thing, The armoured cable bringing the supply in to my Main Distribution box is a 25 metre long run, 2.5mm cable.

This is providing power for the following:

1 X Copeland Scrolll , Running Amps 12.5 amps
Starting is 89 Amps

1 X Carrier Compressor on a Reefer Unit
Running amps are 13 Amps

1 X 4 Hp Le Unite Condensing Unit
Running amps are 5 or 6 amps

Without fans etc I calculate with all three running. at nearly 32 amps

Add to this fans, lights, etc

So safe to assume another 5 or so amps

Surely one 2.5 mm Armoured , on a 30 metre long run from Main Distribution Box in Basement is probably undersized?

Ive recommended to customer he upgrades to minimum 6 mm armoured

I got it priced up, electrician quoted 350, 00 to do the job, but Customer says no,

Id like to know what the regs say

Thx for your help once again

And.........

Frank............You need to tell us about your holiday !!

Did you see Chemi ????
:)

frank
05-01-2006, 11:08 PM
Frank............You need to tell us about your holiday !!

Did you see Chemi ????

Hi Abe

Chemi popped down to the hotel to see us on 29th.

Israel - very nice place, friendly people, Eilat - fantastic! Red Sea - crystal clear, warm, beautiful :)

had a great time, sunshine, clear blue skies every day about 20/22C, warm enough for t shirts. Tel Aviv looks very nice and apparantly is the place to go for nightlife but at my age I like to retire this side of midnight ;) . We only flew over Jerusalem as time was tight but the desert looked great from above. The hotel was 1km from the border with Jordan.

It's a little un-nerving when the guy searching your bags etc when going into the shopping centre (wife likes to shop :( ) has an Uzi 9mm slung around his neck :eek:

All in all a very nice place.

Argus
06-01-2006, 12:22 AM
Without fans etc I calculate with all three running. at nearly 32 amps

Add to this fans, lights, etc

So safe to assume another 5 or so amps

Surely one 2.5 mm Armoured , on a 30 metre long run from Main Distribution Box in Basement is probably undersized?





On the face of it you are probably right.

You need to obtain BS 7671 and read all of Appendix 4, (current carrying capacity of cables).
There are a stack of tables that outline the basic ratings of differing cables and instructions on how to apply them.
Table 4DA, for example is a rating for a bog-standard armoured cable. In it a 2.5mm three or 4 core cable on 3 phases, fixing method 1 (clipped direct) has a rating of 25 amps. This goes up to 26 amps if it is laid on ventilated tray. At this point, you have not factored in voltage drop, operating temperature etc. I would expect a cable run through walls with insulation etc and with the rating factors applied to be nearer 20 amps. But you must work it out for yourself if you have doubts.
I would not advise diversity on a non-variegated circuit like that, because you may well encounter worst-case peak load running conditions and therefore prolonged overload.

The standard, BS 7671, begins by stating that it is not a statutory instrument, but that compliance will be deemed due diligence where there is legislation.
In this case there appears to be a breach of the Electricity at Work Act 1989. The most famous clause, quoted in any incident and ingrained in any electrical engineer?s consciousness, is Regulation 5:

?No electrical equipment shall be put into use where its strength and capability may be exceeded in such a way as may give rise to danger.?

At the very least put your concerns in writing to your customer and advise him to seek a qualified opinion.
The penalties include imprisonment. So if he isn?t gay now, he probably will be when he gets out.

But one last thing. Did you install the electrics or are you in any way responsible for them?
If the answer is no, put your misgivings strongly and unequivocally in writing and back away before you are sucked in.

Good luck.

.
________
Harley-Davidson S-125 (http://www.cyclechaos.com/wiki/Harley-Davidson_S-125)

Abe
06-01-2006, 09:08 AM
But one last thing. Did you install the electrics or are you in any way responsible for them?
If the answer is no, put your misgivings strongly and unequivocally in writing and back away before you are sucked in.

Good luck.

.


Thank you Argus

I did not install the electrics, they are prexisting.
I did " hook " up the equipment to the existing supply.

Therefore, I "am" liable to an extent knowing that the 10mm armoured, or 2.5mm cores are not sufficient load carrying capacity under worst case scenario.

But, I did advise this week to the customer to upgrade to 4mm cores, and IECC Qualified Electrician quoted £350.00 to put supply in.

Customer says, no , we have had no problems in the past, so we will leave as is.

To cover my side, I feel now that I must put it in writing
In the event of fire, I dont want Fire Inspector asking me questions.

Abe
06-01-2006, 09:13 AM
Hi Abe

Israel - very nice place, friendly people, Eilat - fantastic! Red Sea - crystal clear, warm, beautiful :)

All in all a very nice place.


Frank

Pictures look absolutely beautiful, heightened by the fact that we are in the depths of our grimy winter.

Glad you enjoyed yourself, and Ahh well.........back to the old grindstone

:)

Abe

Abe
06-01-2006, 09:29 AM
Argus

Ive been searching for info on BS 7671 Appendix 4

Ive realised, I really need to get my own book

Could u recommend a decent book that is informative, explanotory and comprehendible?

Any suggestions would be great

Thx

Argus
06-01-2006, 10:01 AM
Argus

Ive been searching for info on BS 7671 Appendix 4

Ive realised, I really need to get my own book

Could u recommend a decent book that is informative, explanotory and comprehendible?

Any suggestions would be great

Thx


Aiyub,

BS 7671 is the previous IEE 16th edition of the wiring Regulations for buildings. It was taken over and published as a British Standard about 12 years ago.

Appendix 4 is part of it and gives details on the rating of cables, including de-rating factors. It is not particularly difficult to follow if you only have a single circuit to contend with.

Appendix 3 lists data on fuse and Circuit breaker characteristics, though the best info on these is from the manufacturers of the devices themselves.
You can get a copy here:
http://www.iee.org/shop/
Though I expect that any decent bookshop will either have a copy or get one for you.


As far as your customer is concerned, I would do the needful in writing, recorded delivery, if I were you and take the extra precaution of faxing it to him and keeping the transmission slip.

It always makes me smile when certain A/C manufacturers in the UK provide a ?fuse rating? in their technical data?? It?s totally meaningless and here?s the reason why!


.
________
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Cofreth
06-01-2006, 02:53 PM
Thought the setting of the thermal overload should be 105% of motor ratings for DOL and 68% of motor ratings for Star Delta?