Dust Extractor Tripping the House RCD?

[Copes]

Hi Guys,

Way out of my area of expertise here, but hopefully someone can give me some practical advice on how to work out what's going wrong, or who to ask for help.

Recently got my new workshop cabin wired up by a local sparky.

On the house unit;

• 100amp RCD
  32amp MCB going to cabin

Armoured cable to the cabin

• Prysmian LV Armoured 6943X 3-Core Cable 4mm² x 25m

In the cabin unit;

• 100amp main switch
  32amp MCB got the ring main
  6amp MCB for the lights.

Today my Axminster FM300BC extractor arrived, (with my new tablesaw, but sadly that's way to heavy for just me to build so i can't use it to test the cabin. Going to have to wait for some extra hands on that)
Built it no bother, plugged it in and it started up and after about 30 seconds turned it off. Decided to try again with the doors closed for the noise and it started up but after about 10 seconds just turned off. (It had tripped the house's RCD) So found it had done that, reset it. Tried it again and did the same thing.

Spoke to the electrician and he said it was likely the extractors motor causing the problem, as the draw should only be 6.5amps which wouldn't be a problem.

I did think about lugging the extractor down to the house, and trying it on the mains ring there... is that a sensible step to narrow it down?

(The electrics have been fine using my festool CTL26/power tools)

I'll give axi support a call but they aren't open until monday.

Thanks Guys
#nightmare-as-usual!

 

[kevinlightfoot]

Hi I am no sparky but I had the same problem mostly when the weather was cold my electrician fitted a c curve mcb and the problem was solved hope this helps.Kev.

 

[flh801978]

I would first try the new extractor on the house ring main..even on a decent extension lead to start with...
If it still trips then it's the new machine
A RCD only trips with 30mA or more going to waste so it shouldn't trip with motor startup current

Ian

 

[Monkey Mark]

Just to clarify, is it the MCB or RCD that's tripping?

 

[Copes]

It's the RCD.

All of the mcb s stay fine, flip the rcd and we're back to normal.

 

[deema]

There is a fault to earth with the motor / wiring if the RCD is tripping

 

[Copes]

Thanks guys, I'll try it out on an extension tomorrow then log it down to the houses needs be.

Hopefully it is the motor and not the workshop wiring that's at fault.

Will let you know.

 

[Monkey Mark]

Ok, i think the key here may be "started up and after about 30 seconds turned it off". Is it by any chances a capacitor start motor?

It may be caused by the motors centrifugal switch as it reaches optimal rpm. As it switches it can cause spikes/imbalances due to the capacitor. It could also be transients from a larger startup current being enough.

This can make it a little tricky though as the easy way around this would be a time delayed rcd however this would affect the safety on all circuits unless there were some alterations made depending on how you are with ekletrikity.

 

[sunnybob]

Take your household vac to the shed and run it.
If it trips, call the sparky.
take your new extractor to the house, if it trips, call axminster.

I wouldnt expect a capacitor start motor to take as long as 30 seconds to reach speed.
If you feel capable and safe to do so, you could check the tightness of the electrical connections on the rcd. Its very common for a loose terminal screw to cause this kind of problem. If you are not sure, DONT do it.

 

[Copes]

Sorry didn't get a chance to update this yesterday.

• 
  Tested the extractor in the house, and it had the same issue.
  Tested a whole bunch of things, festool, kettle, various power tools all together in the cabin without any issues.

So i thought surely it was the motor/extractor.

I spoke to Axi this morning and i'm now possibly more confused than i was. The chap on the phone said it was because i potentially had a type B MCB on the house end of the armoured cable, I'm pretty sure the workshop has a type C MCB on its consumer unit, surely if it was because of the (presumed) type B, the MCB would be tripping and not the RCD? Related to this he said that the 1.5kw motor would have a 30amp draw spike on start up, which seemed really high? as surely its running at ~6.5amps usually?

I'll admit i'm really not that savvy with electrics, but the above to me doesn't seem to make sense but you guys seem to grasp the whole concept alot better than me.

In the end they suggested, checking that both MCB's are type B and if they're not getting a sparky out to change them to those.

Does that sound like the best course of action?

Cheers

 

[DTR]

I spoke to Axi this morning and i'm now possibly more confused than i was. The chap on the phone said it was because i potentially had a type B MCB on the house end of the armoured cable, I'm pretty sure the workshop has a type C MCB on its consumer unit, surely if it was because of the (presumed) type B, the MCB would be tripping and not the RCD? Related to this he said that the 1.5kw motor would have a 30amp draw spike on start up, which seemed really high? as surely its running at ~6.5amps usually?

Most motors cause a current spike on start up, but it only lasts for a second or two, then it drops down to the nominal (6.5A in your case). The type of MCBs on your cabin are irrelevant if you're having the same issue when it's plugged into the house. My money is on it being a motor with an earth fault.

 

[Monkey Mark]

Although the intake current is higher, so he's correct on that bit, it would indeed trip the mcb rather than the rcd.

Axi obviously didn't listen too carefully to you. You're too far away for otherwise i would have tested the extractor for you.
There "may" be a fairly simple solution but you'd need a spark to do this for you if you are not competent. What type is your main board? Post a pic if you're not sure.

 

[Copes]

I'll post a picture tonight when i get home of both the main board and the workshops.

I did speak to my sparky who setup the workshop to start with, he suggested to improve the electrics, it might be worth installing at type C RCDO (presuambly in the workshop?)

Cheers
Mark

 

[Copes]

Hi Guys,

Here's the board in the house;
http://imgur.com/Ee7WpxV

and here's what is in the workshop;
http://imgur.com/liaylNx

Actually looks like they're both B type MCB's now i've looked properly, I'm sure i had the conversation with the sparky about type C but oh well.

Is this what you need Mark? Ta

 

[woodpig]

Hi I am no sparky but I had the same problem mostly when the weather was cold my electrician fitted a c curve mcb and the problem was solved hope this helps.Kev.

I spoke to a sparky about this as well and he suggested a C type. Sadly he's not been back to fit it ... :roll:

 

[HappyHacker]

If the MCB is not tripping changing it to a type C will not make any difference, using a C type makes it more resistant to surges but you would need to talk to your electrician to make sure that the Zs values of the wiring can meet the required trip times in the event of a fault.

A 30mA RCD will typically trip at 20-25mA. There may already some leakage from the house electrics/appliances and the extractor motor may just be pushing it over its trip value although it is unusual for it to happen after a fixed time such as 30 seconds. Computers and many modern appliances have a small level of leakage which will produce a typical standing leakage in a house of 10-15mA when all are switched on.

Finding cause of RCDs tripping can be very different and time-consuming. Start by turning off all appliances and unplugging all those that can be unplugged. Turn off all the MCBs in the house and garage that are not powering the extractor and try it again. If the RCD trips either the RCD is overly sensitive or the extractor is faulty. If it does not trip the RCD leave it running while you turn each MCB back on again and if one causes it to trip. If there is no trip try plugging/turning on all those things normally on and see what causes a trip. If this identifies something else causing a problem then leave it unplugged.

Your electrician may have a meter sensitive enough to read the mA leakage to help identify the problem and be able to test the extractor to see if there is an obvious problem.

Having one RCD on the house is not normally recommended and modern practice is to have at least two so there is not a single point of failure and any leakage from electronic items is split across more than one RCD. The use of RCBOS (RCD & MCB combined) for each circuit is also becoming common as the prices have come down.

I recommended unplugging as a Neutral-Earth fault can cause tripping and the Neutral is not normally switched when an item is turned off.

Good luck
Kevin

 

[Monkey Mark]

Ok, so there are obviously many opinions on here so it's up to you to determine your course of action.

The post above shows a good method for narrowing down a cause.

However, should that not resolve your problem, there's another thing you can try, but only if you feel competent.

I still think the cause may be the imbalance caused by the cap or centrifugal switch.

Screwfix sell an 80a 100ma rcd for £20. You could buy one and try it & return if no good. (only 4 wires & straight swap so easy to change) But the extra ma's may be enough to overcome any startup imbalance.

 

[Myfordman]

How can there be any possible imbalance during starting unless there is a leakage path to earth?

 

[Monkey Mark]

How can there be any possible imbalance during starting unless there is a leakage path to earth?

In two ways.
Firstly due to the capacitor. The small time the cap takes to charge can be enough time of power in with no power out to trip the rcd.
Secondly if the motor uses a centrifugal switch with a seperate line & nuetral contracts then both contacts must make at the same time, even a delay of mili secods can cause enough of an imbalance to trip.

There are other reasons too but usually they are on used motirs not new.

It doesn't need to be a leakage to earth just an imbalance between line & neutral.

 

[n0legs]

Screwfix sell an 80a 100ma rcd for £20. You could buy one and try it & return if no good. (only 4 wires & straight swap so easy to change) But the extra ma's may be enough to overcome any startup imbalance.

That's not the best advice Mark I'm sorry.
A 100ma RCD is best kept for the installations that really need them. Sacrificing the safety level a 30ma RCD provides for one item of kit, which seems to have a fault IMHO, is not the way to go with this issue.

To the OP, send the extractor back.