Tool trips RCD intermittently

[RainbowsParticipant @rainbows]

Bought a flex shaft grinder and was happily using it but then 15 minutes into it the RCD triggers. Take it as a fluke and try again, after about 5 minutes it trips again.

Checked continuity with the earth pin and other pins of the plug and found nothing. The fact the tool runs fine for a while also seems odd. Only has a 71W motor so can't be a case of overloading the fuse in normal use.

Anyone know what other tests I can do to find the issue?

[RainbowsParticipant @rainbows]

[John HaineParticipant @johnhaine32865]

There can be problems with suppressor capacitors on tools connected from the live & neutral to earth, as this bleeds a small current out of the live side not balanced by the neutral (which should be at or near earth potential). An RCD which is oversensitive, or possibly has another appliance connected with a leakage too, may then trip. So try disconnecting everything else connected through the same RCD, and test their continuity too. (Though really you need a megger for testing.)

[SillyOldDufferModerator @sillyoldduffer]

Could be the grinder is faulty or – quite likely – you have several slightly leaky devices plugged in and the total is enough to trip the RCD. It is common for devices like motors, TV sets, Microwaves etc to be fitted with filters that leak a little; not a problem unless you have too many of them.

One way to check is to unplug everything in your house apart from the grinder. Then if switching on the grinder pops the RCD it must be faulty. Or take it to a neighbour – if it pops their RCD as well as yours it's probably faulty.

A better way is to buy or borrow an earth current clamp meter. They allow you to measure earth leakage device by device which makes it easier to pin down any that are leaking excessively.

Dave

[Les Jones 1Participant @lesjones1]

Using an insulation tester test between the earth terminal and the live terminal and then between the earth terminal and the neutral terminal. For these tests the trigger must be held in the on position. If it is a modern insulation tester it will probably let you select the test voltage. If so select the 500 volt range. If it is the old fashioned Megger insulation tester they just use 500 volts. Report the reading you get. Is the tool metal cased or all plastic ? Is the mains cable 2 core or 3 core ?

Your staement "found nothing " is not helpful It could mean that you found zero resistance or it could mean you found no continuity which means infinite resistance.

Les.

[EmgeeParticipant @emgee]

This grinder is most likely Class 2 so will not have an earth connected, a PAT tester will no doubt indicate leakage and magnitude.

Best to return the grinder for a replacement.

Emgee

[Brian Sweeting 2Participant @briansweeting2]

With regard to the various leakages that happen with motors you should really use motor rated MCBs.

Look at type C MCB.

 

Edited By Brian Sweeting on 21/02/2018 19:07:27

[Simon Williams 3Participant @simonwilliams3]

Posted by Brian Sweeting on 21/02/2018 19:02:21:

With regard to the various leakages that happen with motors you should really use motor rated MCBs.

Look at type C MCB.

Edited By Brian Sweeting on 21/02/2018 19:07:27

An MCB is for protection against a fault between live and neutral, causing an overcurrent fault. The OP has reported the RCD tripping, so that indicates a (much lower current) leaking from live OR NEUTRAL to earth.

The "type " of MCB (A, B, C or D) only determines the relationship between through current and tripping delay time. So a type C MCB is intended to cope with the start inrush current of an induction motor.

At 71 watts this is likely to be a brush (universal) motor, and needs to be protected against leakage to earth by an RCD (30 mA sensitivity) AND an over current device, typically an MCB. It's no doubt intended to be plugged into a 13A socket fed on a ring main, so there will be an appropriately rated overcurrent device at the consumer unit to protect the ring main wiring, and a (not more than) BS1363 HRC 13A plug fuse in the plug top. Arguably you could reduce the size of the plug top fuse – it should be sized to protect the mains lead. At 71 watts there is no legal requirement for a motor thermal overload type protection.

It's more than likely that the real problem isn't the flexy-shaft motor at all, though dodgy suppression capacitors inside won't help. Intermittent RCD tripping is usually the result of a number of small leakages in different appliances which sometimes erratically add up and out she pops. SOD has it above. But if the tool is is new it shouldn't have a problem so take it back. If the new one also does it, then it's more than probable the OP has got another fault as well (maybe more than one) and the proper course of action is to call in someone who is competent to find the fault(s). It's far from straightforward to do, and needs someone competent to design the requisite tests and interpret the results.

If it is a class 2 device (double insulated, only two conductors in the mains lead) and it is tripping the supply RCD even occasionally quarantine it immediately, it's dangerous.

​HTH Simon

[Ian PParticipant @ianp]

Posted by Brian Sweeting on 21/02/2018 19:02:21:

With regard to the various leakages that happen with motors you should really use motor rated MCBs.

Look at type C MCB.

Edited By Brian Sweeting on 21/02/2018 19:07:27

There are motors and there are motors.

Not many households have motor rated MCBs for the sewing machines, food mixers and ventilator fans etc.

Ian P

[John RuddParticipant @johnrudd16576]

 

Posted by Ian P on 21/02/2018 20:10:34:

Posted by Brian Sweeting on 21/02/2018 19:02:21:

With regard to the various leakages that happen with motors you should really use motor rated MCBs.

Look at type C MCB.

There are motors and there are motors.

Not many households have motor rated MCBs for the sewing machines, food mixers and ventilator fans etc.

Ian P

Ahh some sense at last…….

My new house is less than 6 months old….is fitted with a split load dis board….all mcb's are type B…..

During the time I've lived there I've used a table saw rated at 1600w fitted with a brushed motor, various circular saws rated at 1200 watts or more, a tile cutter with an induction motor rated at 500 watts…..none have tripped mcb or rcd devices…

In fact the only time an rcd has tripped,has been when I was modifying some wiring in the garage ( dead work only isolated at the dis board by switching off the relevant mcb feeding the garage) , I inadvertantly touched an earth wire to an adjacent neutral conductor in a light fitting….

With regards to Simon's post above, if the flexy grinder device is double insulated, it wont have an earth, ergo it will not trip an rcd as there is no earth connection surely?

Edited By John Rudd on 21/02/2018 20:58:29

[Simon Williams 3Participant @simonwilliams3]

With regards to Simon's post above, if the flexy grinder device is double insulated, it wont have an earth, ergo it will not trip an rcd as there is no earth connection surely?

Edited By John Rudd on 21/02/2018 20:58:29

Thank you John, I had to think twice about this one when I was typing my post.

Imagine (heaven forfend) that there was a path from either supply conductor to the exposed conductive parts of the device. For example, it's a flexy grinder so one assumes the flexy drive cable is made of metal, and likely has a chuck on the end. With a fault from the metal to live, if you touch the exposed metal, you get a shock off it. Hopefully the RCD does what it should and disconnects the supply promptly. The rcd is sensitive to the imbalance between the live and neutral currents, so if some of the supply current returns to the source (actually the star point on the substation transformer) via earth it pops out. You don't need an earth conductor in the supply cable for this to work.

I agree that the concept of a double insulated tool is that exposed conductive parts are insulated from the supply – by two independent layers of insulation. It is also a requirement of the double insulated approval that an earth conductor CANNOT be brought into the double insulated area, that's why the cable only has two cores. But there remains a remote but worrying possibility that the insulation has broken down and that the exposed metal parts have become live (or neutral).

It's very unlikely, double insulation is a very good protection. But if it has happened, however inexplicably, the tool is dangerous and needs dumping. So if a double insulated tool is blowing out the RCD supplying it then this indicates that there could be something really nasty going on.

As I understand it the tool is fairly new, so one would hope that of all the faults that might occur this one is way down the list of probabilities. But IF the OP is seeing an RCD trip with a double insulated tool this is the only mechanism I can see that would cause this, and it spells trouble with a capital ouch.

This is all conjecture; we could really do with an answer to Les' post above saying what the OP measured and found, also what is the tool (model no etc) and how many cores it has in its mains lead.

HTH Simon

[Ed DuffnerParticipant @edduffner79357]

It would be useful to know the make and model of grinder, a picture of the information panel perhaps – if possible. Is it a mains voltage power tool or plugged into a low voltage transformer or proprietary power supply? Also which country is the OP located and what voltage does the kit run on?

Just to be pedantic if I may, …Ring mains are the power lines that distribute electricity around the country, ring 'circuits' are used in domestic and similar type fixed wiring – at least this what we were taught in college.

Cheers,
Ed.

[John RuddParticipant @johnrudd16576]

Posted by Ed Duffner on 21/02/2018 21:55:13:

It would be useful to know the make and model of grinder, a picture of the information panel perhaps – if possible. Is it a mains voltage power tool or plugged into a low voltage transformer or proprietary power supply? Also which country is the OP located and what voltage does the kit run on?

Cheers,
Ed.

Ed,

Rainbows lives in Sheffield……. So safe to say the grinder thingy runs on UK based lecktrickery either directly or thru some gizzmo that he thinks is causing him grief

[Simon Williams 3Participant @simonwilliams3]

Ed – no you're not being pedantic at all, actually made me chuckle. My fault for being lackadaisical with my terminology. Thank you for putting me right!

I'm guessing that the grinder in question is the one Rainbows has bought in response to his advert (classified) just now. As such it may be my assumption it is fairly new is unwarranted. Anyway, we could get further if – as Ed suggests – we had sight of the motor rating plate.

Rgds to all

Simon

[RainbowsParticipant @rainbows]

Double insulated? Warranties? Fairly new? Certainly not :v

Is a Sew-Tric motor thats probably older than me.

Had a go of unplugging all the other possible leaks in the circuit but it still got the RCD for a third time.

Seller was nice enough to let me return it though, will probably bite the bullet and buy a new chinese machine

[Gray62Participant @gray62]

Rainbows, These from Axminster are worth a look at. Had one for several years along with some of the additional handpieces. Similar to the US made foredom but on a tighter budget.

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