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 Post subject: Dewalt 745 tripping MCB
PostPosted: Sat Jul 28, 2018 5:30 pm 
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Hi

I just had a dewalt 745 table saw delivered, I bought it from Amazon warehouse in a "damaged box but like new" condition. Looking at the box it was obvious it had been opened by someone rather than just being a damaged box. It was delivered while I was out otherwise I wouldn't have accepted it.

Anyway I set it up on the kitchen floor (blade all the way down) to test it before contacting Amazon. When I switched it on the motor started for a second before the MCB tripped. Took it outside to a dedicated socket on own radial, same thing happened again.

Manual says it's 10amp so I am assuming it should be fine on domestic circuits? Does it look like this one is faulty?

Thanks
AH


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PostPosted: Sat Jul 28, 2018 5:51 pm 
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Until someone more knowledgeable than me comes along
https://forums.overclockers.co.uk/threa ... .18766624/


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PostPosted: Sat Jul 28, 2018 6:46 pm 
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boxedin wrote:
Until someone more knowledgeable than me comes along
https://forums.overclockers.co.uk/threa ... .18766624/


Hi - I saw that but there seemed to be a lot off guess work in the replies.

I've had a lidl table saw and cement mixer (110v) running on the outdoor circuit without issues.


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PostPosted: Sat Jul 28, 2018 10:02 pm 
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similar thread here
dewalt-table-saw-that-doesnt-work-properly-t95134.html

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PostPosted: Sat Jul 28, 2018 11:08 pm 
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ahfix wrote:
Hi

I just had a dewalt 745 table saw delivered, I bought it from Amazon warehouse in a "damaged box but like new" condition. Looking at the box it was obvious it had been opened by someone rather than just being a damaged box. It was delivered while I was out otherwise I wouldn't have accepted it.

Anyway I set it up on the kitchen floor (blade all the way down) to test it before contacting Amazon. When I switched it on the motor started for a second before the MCB tripped. Took it outside to a dedicated socket on own radial, same thing happened again.

Manual says it's 10amp so I am assuming it should be fine on domestic circuits? Does it look like this one is faulty?

Thanks
AH
It should run ok of your ring main have you check the plug in case it's cross wired

Sent from my SM-G960F using Tapatalk


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PostPosted: Sun Jul 29, 2018 7:21 am 
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big-all wrote:

That was specific to 110 volt, B-A, but there is generally a bit of an issue running large motors (the DW745 is 1850 watts) on domestic ring mains caused by the type of MCB in the consumer unit not being optimal for motor use. The DW745 motor is rated at about 10A on 230 volt but that means it will probably have a starting load of 20 to 30A (based on experience of the type) - or at or even over the trip rating of a 20A type B MCB. Ideally any motor circuit should have a type C MCB ("motor rated") which will handle the high startup current draw as opposed to the "normal" type B installed in most domestic set-ups. I do think it might be as well to have a sparkie take a look at the circuit before swapping out the MCB to be on the safe side. The comment about the massive initial current draw is very true for such a large motor, but the type D someone referred to is surely only seen on circuits supplying equipment such as compressors which have large motors starting up under near stall (full load) conditions

Not a sparkie, but lived with the issues around large (5 to 6.5kVA) 110 volt transformers and large motors on static equipment for years, so comments based on personal observations only.

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PostPosted: Sun Jul 29, 2018 1:50 pm 
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I''ll have a guess that the motors used, as with many modern motors are very low on "iron" in the laminations around the windings. That's where the magnetism is induced to turn the motor. It makes them physically light, (and cheaper) but very heavy on starting current, and they're sometimes referred to as "short circuit start" because they are, for split second virtually a "short circuit". That and the relatively high current while spinning up to speed will pull the "wrong sort" of MCB out.



For this message the author Dave54 has received gratitude : Job and Knock
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PostPosted: Sun Jul 29, 2018 3:39 pm 
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Thanks for all the replies.

Last night I plugged the saw into the ground floor ring main, for noise reasons I only switched it on for a few seconds but it did spin up and ran until i physically switched it off. No trip. Both radials I initially used for tests were 16amp, so it would seem it is a start-up current/load issue.

The CU has a couple of spare 32amp Type-B's, I guess my options are:

1. Replace the outdoor socket with one of the spare 32amp MCBs with the appropriate sized T+E cable (sparkie job)

2. Install a Type-C MCB at 20amps - however seems like an unnecessary expense if option 1 is viable

3. Go with the Bosch GTS 10j which has soft start, avoiding the MCU issue altogether and making it more flexible if I end up using the saw elsewhere.

I was torn between the Bosch 10j and the Dewalt 745, but went with the Dewalt for the larger rip capacity (420mm vs 640mm I think). To be honest I am not sure how much I'll use it yet, there have been a number of time i wanted to rip something or do a job but abandoned it. I have a Parkside table saw which I have hardly used, mainly because it was a faff to set up the guides and make them square etc, so I wanted something that would allow me to do the majority of DIY jobs with minimum set-up resulting in me using it more often.

AH


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PostPosted: Mon Jul 30, 2018 11:55 am 
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So I called Dewalt technical support ... getting information out of them was a bit like pulling teeth (is that usual?) but the chap told me they haven't heard about the saw tripping breakers and can be used on domestic circuits :dunno:

Should I believe him? It is worth ordering another and trying it?

I am sending the one I have back anyway as looks like it was ex-display or something not "new" in damaged packing as advertised.

AH


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PostPosted: Mon Jul 30, 2018 9:31 pm 
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ahfix wrote:
Last night I plugged the saw into the ground floor ring main, for noise reasons I only switched it on for a few seconds but it did spin up and ran until i physically switched it off. No trip. Both radials I initially used for tests were 16amp, so it would seem it is a start-up current/load issue.

At the risk of saying "I told you so.....". What that tells me is that your wiring - what you want to run this tool off - simply isn't up to the task. The saw may well start up in the kitchen, but a domestic ring main will often have 2 no x 2.5mm2 3-core cables (or in crude terms 5.0mm2) - a spur will often be 4.0mm2 cable and that combined with the extra length of run will result in voltage drop. Have you tested the voltages at the spur end and in the kitchen for comparison? In any case a startup test is only part of the story - when the saw is running and you are making heavy cuts I feel that no amount of soft start technology on a universal motor will prevent the motor getting hot and tripping or of drawing too much current from the MCB and tripping that. I very much doubt that buying the Bosch will make any difference because the problem appears to be in your power supply and that's where I'd start.

ahfix wrote:
So I called Dewalt technical support ... getting information out of them was a bit like pulling teeth (is that usual?) but the chap told me they haven't heard about the saw tripping breakers and can be used on domestic circuits :dunno:

Should I believe him? It is worth ordering another and trying it?

I wouldn't believe him. Personally I think he is just looking at the specs and quoting from a sheet. I've tripped quite a few saws over the years on 110 volt - the current generation of motors allied with the much more sensitive types of MCBs we seem to have nowadays has only made the situation worse IMHO. The fact is that in an ideal environment the DW745 should and will run off a 20A type B MCB, providing that the mains cable is of optimal size (preferably oversize to counter the distance/voltage drop scenario which is the result of increasing resistance in the copper wire over distance), that there are no other heavy loads on the circuit, that the plugs and sockets are of adequate size (a 13A plug has a lower rating than a BS.4343 16A blue plug because the contacts, etc are far smaller in size and that alone can also add unwanted to resistance in the circuit) - and even with all that you are still running near to the edge because of the size of the motor and what happens with it when you load it heavily.

Were it me, I'd be thinking in terms of testing the voltage at the end of the spur, of putting the shed on a separate spur/MCB with a 32A MCB and the cable increased to 6.0mm2 or even 10.0mm2 (as you would do for a power shower) and swapping tyo a BS.4343 type blue 230 volt plug (the type they use for caravan connections precisely because they are weatherproof to a degree, but they are also far more robust than 3-pin 13A plugs and they have much bigger contacts (word of caution, though, they do NOT have a fuse protection so your CU must be appropriate)

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For this message the author Job and Knock has received gratitude : ahfix
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PostPosted: Tue Jul 31, 2018 1:21 pm 
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Thanks for the reply, really useful.

I'll hold off buying anything until I have had a think about the electrics and spoken with a spark.

Cheers
Ah


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PostPosted: Mon Aug 13, 2018 12:55 pm 
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Thought I'd feedback so others can benefit from my findings.

I haven't done anything about the electrics yet but I got itchy and bought the Bosch GTS 10j :cheers:

Anyway, the soft start has solved the problem, no trips and has been working on a 13 amp extension (with RCD) with the Titan shop vacuum running at the same time.

I've only been cutting ply but so far so good, gets up and running until get the electrics sorted out.

ah


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