Earth Loop Impedence Test Equipment

[Brains]

Re: Earth Loop Impedence Test Equipment

Simonb

Be very careful if you allow the insulation to fall to 0.25Mohm. You can affect the complete installation. Also, if the appliance is connected to an RCD controlled circuit and the insulation failure is neutral to earth, the RCD will not reset causing the user to go ballistic. (Had a problem like this a couple of weeks ago).

The minimum insulation value allowed by IEE Wiring regs (for an installation) is 0.25Mohms. It is generally accepted that if the total insulation value at a bus-bar of a Consumer Unit is less than 2Mohm, investigation should be undertaken.

Hate to teach you to suck eggs, but, insulation failures can lead to fires.

Also, be careful with the soft tester as you describe. Key to popping the fuse or tripping the MCB within the required disconnection time is the value of the earth fault loop impedance. In other words, use a decent tester (I know they cost the earth but what choice is there really 😉 )

[simonb]

Re: Earth Loop Impedence Test Equipment

thanks for reply mate, agree with everything you said did not know that minimum was 0.25meg for complete installation, as a matter of interest is this reading taken from consumer unit main earth to live and neutral? and does this reading take into account of appliances switched on or off on ring or cooker circuit?

anyway if you repair an appliance i think we should just be responsible for just the appliance as if customer buys new and installs appliance, ie plug it in the manufacturer is not responsible for the dodgy socket that may be wired up wrong? although personally i check.

i honestly think that repairing appliances is a dodgy game to be in although i now its probably unusual for something to go drastlically wrong,
who can honestly say they check every single wire on an appliance after repair, i used to refurbish in factory and strip right down to check, like on hotpoint wm take front off and check wiring and have seen some right dodgy wiring jobs, so if you just fit brushes whos to know whats bodged elsewhere

and who is responsible if something happens – the engineer who was last on job i am led to believe(grey area?) yes?

u take your car to garage to have a new starter motor fitted and the brakes go and you crash who is to blame? the driver for not having them serviced?

u have a new motor fitted to your tumble dryer and the thing goes up in smoke due to another problem a day later who is to blame? the engineer?

simon

[kwatt]

Simon,

The last three or four points there are worthy of a healthy debate all of their own and it is an interesting debate indeed. 😉

K.

[Brains]

Re: Earth Loop Impedence Test Equipment

Wow – look like I opened a can of wiggly worms!

The 0.25Mohm is in any combination measuring across the different bus-bars at the Consumer Unit. This limit value is based upon the fact the all the circuits are in parallel. So, if one or more are down in value, clearly it impacts the whole installation. I always measure from the bars to earth then investigate the individual circuits.

Typically, if I am doing a periodic (electrical installation) inspection, I switch on the appliances & carry out an insulation check . I recently came across a Whirlpool tumble dryer where the element wiring & terminations were at the point of melting & breaking down & the insulation was way low.

You are right in what you say that whoever looks at it last, may be held accountable if something horrible goes wrong. This is why I always ‘eyeball’ the insides of an appliance & record all the test data on a test sheet I put together. I know it takes a little longer, but, I for one, want to sleep at night.

At the end of the day, I guess we have to record all the data just in case something does go wrong. At least, we can then be reasonably ‘comfortable’ that best efforts were applied.

I once had a car whereby a junior idiot in a garage fixed it & the thing failed – went into the back of another. Had to take the garage to court etc with the help of the AA. Another tale for another day.

It is becoming more difficult in the whole electrica industry. oh well…. only another 10 years before I hang up the megger 😆

[Brains]

Re: Earth Loop Impedence Test Equipment

Ooops – made an error. For 230V circuits when testing at 500V lower limit is 0.5Mohm not 0.25Mohm. Only for SELV circuits is the lower limit is 0.25Mohm.

If you look in the latest IEE site Guide page 73, it gives you all this good stuff. 🙄

When carrying out Periodic Inspections, fixed loads & lamps are normally disconnected or switches are opened. Personally, I like to do a quick check in case there is something nasty brewing as mentioned above.

[simonb]

Re: Earth Loop Impedence Test Equipment

thanks for reply mate,

just to add in iee code of inhouse repair and inspection of electrical equipment (think or appliances) it does actually say somewhere that all tests must include a full visual inspection of appliance i think this cannot be fulfilled realistically ie. i know most engineers that work on contract of for manufacturer or even self-employed some days go out with 10-12 or more jobs daily that mean in and out of the house as quick as you can or your not getting home for tea tonight !

so were is the blame? if i could take a decent fee of each repair maybe i could spend longer on job carrying out a full inspection and test as is the legal requirement.

so many customers say well we went to bed and in the morning it was still going round tumble dryers, washers the lot

i know i wouldn’t, alot of responsibility i think for tuppence!

buy the way how to you get them fancy pictures up on left hand side think ill get one of cowboy! howde!

simonb

[Brains]

Re: Earth Loop Impedence Test Equipment

Simonb

Thanks for the reply… so just following another thread (not generally associated with loop testing as such)…. does the manufacturer of any (washer / tumble dryer etc) machine specify that they can be run unattended or at night? We have a dishwasher from a well known manufacturer that has a timer on it. Mrs Brains normally sets it to work at about 3am so that we can benefit from cheaper electricity (economy 7). (As you probably can gather, I normally don’t repair wet goods – I focus on electric cookers, microwaves, electrical installation etc).

[shane]

Re: Earth Loop Impedence Test Equipment

Brains wrote:Simonb

Thanks for the reply… so just following another thread (not generally associated with loop testing as such)…. does the manufacturer of any (washer / tumble dryer etc) machine specify that they can be run unattended or at night? We have a dishwasher from a well known manufacturer that has a timer on it. Mrs Brains normally sets it to work at about 3am so that we can benefit from cheaper electricity (economy 7). (As you probably can gather, I normally don’t repair wet goods – I focus on electric cookers, microwaves, electrical installation etc).

We would discourage any client from using an appliance on off peak supply during the night, as a lot of people operating in this way use “cheapo” time switches which are of very “iffy” construction and a potential fire risk. There is a much greater probability of a flood due to a hose bursting, due to higher water pressure in the “quiet hours”, or damage from rodents that tend to do their dirty work in the dead of the night. We had a case of a detached country house where mice chewed through a fill hose when the house was unattended. Due to rotten floorboards under the appliance, the resultant flood ran into the cellar
below and flooded it to a depth of four and half feet. We replaced the hose but the blighters repeated the exercise a few weeks later.
Shortly afterwards the client move house.

All taps should be turned off when the appliance is not in use ! This used to be a standard warning in instruction books from one well known
manufacturer.

shane.

[Brains]

Re: Earth Loop Impedence Test Equipment

Shane

This is what I thought. I am well aware of the cheap timeswitches. However, the timeswitch is integral with this particular dishwasher (not a plug-in cheapy)- By virtue of the integral timeswitch the manufacturer is encouraging off-peak use – any comments?

[sparkey]

Re: Earth Loop Impedence Test Equipment

If your machine is one of the better quality appliances it will probably have several safety features built in, ie: anti flood devices in base of machine, electronic controls monitor filling stages so if water valve is energised for more than four minutes programme will abort and machine drain out and stop, this does not apply to many of the cheaper made machines. You pay’s your money and take your chances.
Sparkey

[Brains]

Re: Earth Loop Impedence Test Equipment

Understood – thank you. I always continue to learn something about this industry. 🙂

Thanks for shedding light on this. The Dishwasher was certainly not one of the cheapies I see regularly featured on Whitegoods.

[simonb]

Re: Earth Loop Impedence Test Equipment

hi again

i know thread has changed again but went out to hotpoint machine late wm did a repair on machine and insulation was at 250k turned out to be motor as normaly is

as a matter of interest did a few calaculations maybe someone can shine a light on this

the earth leakage on this machine would be(working on 230/250000)

0.00092 amps – this would be present on all metal parts of machine traveling through earth connector to plug top and then on to neutral (most urban houses use neutral as earth return)

and lethal shock level i believe is between 5 and 30 (0.0030) milliamps depending on bodys resistance to current and other factors at time of shock

this is where my electrical knowlege gets stressed!
i know if you get a dead short the fuse on plug top will blow outright fast but when will it blow with low-insulation in theory anything less that 17 ohms will exceed 13 amps and i know depending on fuse capabilities on over current this should blow around this ohm level.(when would rcd pick up on fault?)

basically what im getting at is iv had a lot of machines that are less than 1 meg i always go on the side of caution but sometimes loose the job as customer wont pay for new motor etc.which is a sickner

if you can pinpoint the low insulation fault to a componant ie. motor and not wiring are you pretty safe at a given value less than 1 meg (but i know this is an indication of problems to come) if earth loop test checks out ok.whats the worst that can happen the fuse will blow??

any comments

simon

[Brains]

Re: Earth Loop Impedence Test Equipment

Simonb

Sometimes low insulation on a motor can be due to X & Y capacitors that are used as interference suppressors (I don’t know if they are fitted directly to the motor) – so I would also be inclined to check these.

An RCd will generally trip if the leakage current in the cpc conductor exceeds 30mA.

The 0.5Mohm is for the electrical installation. Keep in mind that you are testing with a dc source when using a megger. When ac is supplied to the system, the impedance (as opposed to resistance) can now be different.

The insulation values that we all live & breathe by are set by the IEE.

I took a look at the IEE Code of Practice for In-Service Inspection & Testing of Electrical Equipment. For all class 1 equipment below 3kW the min acceptable value is 1.0Mohm.

So, if you leave an appliance with low insualtion in a useable condition – what happens if it smokes or gives someone a shock? Who is responsible (back to the previous discussion)? Getting the customer to sign a disclaimer may not help as they are not technically qualified. Personally, I would be inclined to disable it in a manner that it can be re-used after repair & write it up on the invoice that it should not be used as it is dangerous.

Also keep in mind that a fuse (or MCB) is there for short circuit protection. For excess current protection, it takes a lot longer for it fail (becasuse more current is required). When a consumer unit fuse fails due to a short, it has to open in a specified period of time (normally 0.4 or 5 seconds depending on circuit type). The time to fail is governed by lots of different things. However, a key parameter is the Earth Fault Loop Impedance. If you take the supply volts & divide it by the earth fault loop impedance this will give you the Prospective Short Circuit Current for the circuit at the point of measurement. Now this is important, since it is this current at short circuit conditions that causes the fuse or MCB to open.

The RCD will open when there is difference of greater than 30mA between phase and neutral currents.

For a ring final circuit, max earth fault loop using a BS EN60898 type B 32A MCB is 1.5 ohms. If you have a radial circuit using a 16A MCB (BS EN 60898 type B) then max earth loop is 3.0ohms (ref. Amtech Ref Sheet 1).

If you consider a 13A (BS1362) fuse (like in a plug top) then max allowable earth fault loop is 2.53 ohms for 0.4 secs disconnection or 4.00 ohms for 5 seconds.

Your 17ohms is only true at 13-amps. You will need more than 13-amps to blow the fuse and only continual sustained 13-amps may make it fail due to overheating rather than a fault. I need to study some pretty horrible graphs (called I*2t characteristics) to define when it fails in excess of 13-amps 👿

As discussed above, disconnection time is the key to protection effectivity.

I hope I have not totally confused you with my ramblings 😈

Personally, if I come across appliance values less than 1 Meg, I want to track down what is the problem (I’m sure you would as well).

Unfortunetly, safety these days is turning into what is known as risk-management. Personally, I like to work to the various codes of practice so that I can err on the side of conservatism. You have to make a choice as to what standards you work to ( I’m sure they are very high). This is all very risk-management orientated.

Hope this helps. Look forward to hearing from you again.

[Author]

Posts