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Old 6th Apr 2018, 9:59 pm   #1
LukeG83
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Default Creda Freestanding electric cooker. 1970s

I have recently had installed a electricity circuit from kitchen to consumer unit for a cooker in my kitchen. The house previously had a gas cooker which died, and I made the decision to go to electric partly in order to utilise an immaculate (looking) freestanding electric cooker from the 1970s.

The cooker was in full working order when last used a few years ago, but has been transported.

A 10mm cable has been installed from fuse box to kitchen via a cooker switch and plug socket. The cooker has its own mcb sharing a rcd with the house power sockets.

The problem I'm having is this. Three of the four rings works when used alone. When used in conjunction with another the cooker trips the rcd. The forth ring did work briefly then tripped the rcd, even if used alone.

The grill and small upper oven work when used alone but trip the rcd (not mcb), when used with another function such as a hob ring. The main oven immediately trips the rcd when used alone or with another function.

The Rcd states 63 A
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Old 6th Apr 2018, 10:10 pm   #2
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Default Re: Creda Freestanding electric cooker. 1970s

Forgot to mention, it is installed to a Rcd as the cooker socket also had a standard plug socket I'm basically wondering if I should up the rating on the rcd that is used
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Old 6th Apr 2018, 10:14 pm   #3
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Default Re: Creda Freestanding electric cooker. 1970s

Hi Luke

Most likely your RCD is tripping due to leakage Live to Earth.

Most domestic RCD's will be rated a 30mA.

A lot of these older elements wll leak live to ground it's what finally did for our old Belling.

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Old 6th Apr 2018, 10:17 pm   #4
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Default Re: Creda Freestanding electric cooker. 1970s

Not wired on a ring BTW..technically it's a radial circuit.

This is typical earth leakage from the elements, the current theory is that they accumulate damp if not used for a while. In fact my dad's recently resurrected Hostess Trolley which was put-away working and not used for around 3 years also took the RCD out intermittently for the first few hours use.

There's a thread on the forum somewhere about the method of driving out the moisture. I would personally run it with one ring at a time to see if the insulation improves with use. It must be the accumulated leakage that's tripping it. One of the nuisances of modern living I suppose!
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Last edited by McMurdo; 6th Apr 2018 at 10:19 pm. Reason: typo
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Old 6th Apr 2018, 10:17 pm   #5
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Default Re: Creda Freestanding electric cooker. 1970s

Hi thanks - so, find and replace the elements or scrap the cooker? I was wondering if I could up the rating on the rcd (given one isn't required if no power socket is on the cooker switch).

I'm desperate to keep this cooker if I can
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Old 6th Apr 2018, 10:24 pm   #6
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Default Re: Creda Freestanding electric cooker. 1970s

Hi Luke

Do you have access to a Megger?

It may not be the elements although they are usually prime suspects in my limited experience, something may have got damp.

You need to find the source of the leakage.

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Last edited by Cobaltblue; 6th Apr 2018 at 10:25 pm. Reason: Partially crossed with Kevin
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Old 6th Apr 2018, 10:25 pm   #7
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Default Re: Creda Freestanding electric cooker. 1970s

Looking on ebay it appears the rings are available and possibly even the oven elects. However, first I will try drying this out by running the working parts of the oven for a few weeks.

It's such a beautiful piece I simply don't want to send to landfill!!
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Old 6th Apr 2018, 10:27 pm   #8
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Default Re: Creda Freestanding electric cooker. 1970s

you'd have to consult an electrician to see if the supply is suitable for non-rcd circuits. There are all sorts of rules regarding earth loop impedance and disconnect times, types of mcb curve used, class of supply/earthing arrangements etc that all have to be considered. RCD's aren't just fitted these days for a bit of convenient safety, they're also there to comply with regulations some of which which are now under law.
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Old 6th Apr 2018, 10:28 pm   #9
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Default Re: Creda Freestanding electric cooker. 1970s

You could try heating up the elements with say a blowtorch or a hot oven to try and bake out any dampness that has ingressed into the powdered aluminium oxide insulator that surrounds the inside core of the rings where the resistance wire is. A 1000 V Megger would be handy here.

Any chance of a photo or 2 and the rating plate as we had an Creda cooker of this vintage. My grandma was on the rotisserie motor assembly line at the Creda factory in Yate.

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Old 6th Apr 2018, 10:30 pm   #10
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Default Re: Creda Freestanding electric cooker. 1970s

If you can find a 24 volt isolating transformer rated at several amps just leave one element on and connect the cooker to it for about 12 hours and then repeat for all the other elements.
I have a feeling that cookers and immersion heaters are allowed to be run without an RCD.
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Old 6th Apr 2018, 10:44 pm   #11
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Default Re: Creda Freestanding electric cooker. 1970s

Picture attached.

Only one ring can stay on long enough to boil a kettle of water.
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Old 6th Apr 2018, 10:48 pm   #12
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Default Re: Creda Freestanding electric cooker. 1970s

Spot the mistake. I thought it was a creda!!!

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Old 6th Apr 2018, 11:38 pm   #13
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Default Re: Creda Freestanding electric cooker. 1970s

I thought it was likely that the cooker had raised solid hotplates as they are well known to trip RCDs.

If that was the case I would have advised you to run them one at a time for a good 10 or 15 min on a non-RCD circuit to dry them out. If a non RCD circuit is not available because you have a single RCD protecting all the circuits (typical 16th edition compliant board), then an alternative would be to temporarily remove the CPC (earth wire) to the cooker for the drying out session. Obviously that is dangerous and must be done under controlled conditions.

However, those old school spiral elements don't leak in quite the same way as the solid ones and I would be looking for dampness or grease causing leakage on the ceramic blocks carrying the spade connectors on the elements or even elsewhere in the wiring.
Running the elements might not heat the affected parts sufficiently to drive out the moisture.

I suggest you disconnect the cooker completely from the supply, and megger between live and earth on the mains terminals switching on and off each element and recording the readings Remove any grease or dirt from between terminals of the elements and switches, apply heat from a hairdryer to these parts and keep rechecking with the megger to see if the leakage reduces or disappears.

A megga is the best instrument because it uses high voltage, but an ordinary digital multimeter set to its highest ohms range will tell you what you need to know in most cases.
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Old 7th Apr 2018, 7:42 am   #14
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Default Re: Creda Freestanding electric cooker. 1970s

If you can get hold of a proper high-voltage insulation tester (even borrowed), that is the best thing for testing your cooker. Ordinary multimeters use a very low test voltage that might not be enough to cause the insulation to begin leaking. But if you can't lay your hands on a Megger, you have nothing to lose by trying an ordinary multimeter anyway. Users of old-fashioned hand-cranked Meggers used to reckon they could feel the difference between good and failing insulation without even looking at the meter needle. (Which is at least plausible; any energy going anywhere it shouldn't must be coming from somewhere, and the only place it can be coming from is the work done turning the crank, which ought to get stiffer as soon as the insulation begins to break down and you start to have to push a current through the fault as well as overcoming mechanical friction.)

It could be that the mineral insulation between the heating wire and the earthed metal ring has absorbed some moisture and become conductive. Applying heat with a blowtorch or hot air gun (since you can't use the heating element to heat itself, unless you could arrange a temporary supply not protected by an RCD) or even just placing in a fire for awhile should drive the moisture back out the same way it got in, and regular use of the cooker from then on will keep it at bay, preventing it from building up enough between successive uses to cause tripworthy amounts of earth leakage current. Alternatively, it could simply be cooking residues deposited on the electrics as the vapour cools, requiring little more than a good clean. Anyway, with an insulation tester, you should be able to trace the fault easily enough just by successively disconnecting things; whatever you disconnected that cleared the fault, must be the culprit.
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Old 7th Apr 2018, 9:53 am   #15
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Default Re: Creda Freestanding electric cooker. 1970s

Users differ a little obviously, my mother used to have a habit with these 4 ring hobs of only using the same two rings, with the result that the back two would accumulate a thin (and almost invisible) layer of what can only be described as 'clod'. With the addition of moisture from a period of storage, the clod being quite hygroscopic becomes a semi-conductive goo- with predictable results if an RCD is in place.

As mentioned a Megger is the weapon of choice. It's a nice looking cooker, totally agree.
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Old 7th Apr 2018, 12:27 pm   #16
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Default Re: Creda Freestanding electric cooker. 1970s

Quote:
n alternative would be to temporarily remove the CPC (earth wire) to the cooker for the drying out session.
I would advise against doing this. If it is tripping an RCD it is passing at least 15mA to earth (lower tripping threshold for a 30mA RCD) which is enough to be dangerous, and if a short develops as the elements are baked out the whole cooker could become fully live. A non-RCD-protected circuit is permitted provided certain criteria are met, as mentioned above, however one that often applies in domestic installations is that in no place may a PVC-sheathed cable be buried less than 50mm deep. Since cooker cables are so often plastered over in the kitchen wall, for this reason if nothing else under the present regulations they are often required to have RCD protection. There is another limitation in that if protection is being omitted because the device has high leakage (>10mA per circuit), rather than to avoid the risk of nuisance tripping (e.g. on freezers) then a high integrity CPC should be provided, with additional requirements for minimum CSA in some cases. And of course permitting leakage from a known cause (damp elements) can also unwittingly lead to tolerating leakage from an unknown one (e.g. charred or damp wiring insulation).

Unsealed sheathed heating elements that have collected moisture often exhibit the behaviour that the resistance falls as they warm up, because moisture from throughout the element is driven towards the colder ends where it condenses in a more concentrated leakage path. Ensuring the ends reach a high temperature, without endangering the connections, may speed the final evaporation of the moisture to the outside. Sometimes the drying process is effective, sometimes not as permanent conductive paths can build up, An isolating transformer capable of powering one element would be a boon if you want to give them their best shot.
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Old 7th Apr 2018, 1:38 pm   #17
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Default Re: Creda Freestanding electric cooker. 1970s

The cooker circuit sharing an RCD with the socket circuits is not ideal because if there are already the normal suppression capacitor currents from washing machine, dishwasher, computers, etc., that doesn't allow much for the cooker.
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Old 7th Apr 2018, 2:41 pm   #18
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Default Re: Creda Freestanding electric cooker. 1970s

I would prefer that the cooker be on a dedicated RCD so as to reduce trips from cumulative leakage in numerous appliances.

Metal sheathed heating elements are very liable to earth leakage and may still trip the RCD.
Try each element on its own, the insulation resistance will probably improve with use as the heat displaces moisture.
Carefully examine the wires and connections leading to each element, sometimes they become coated with carbonised food deposits, these deposits are conductive.

If tripping still occurs than several possibilities exist.

Run the cooker from a 110 volt isolating transformer for few hours. This will confine any earth leakage to the 110 volt secondary circuit and won't trip the RCD in the 230 volt circuit.
110 volts will get it hot enough to dry out any damp.

As a last resort, try a test with the earth wire disconnected from the frame of the cooker, and the frame of the cooker connected to mains neutral instead.
Any leakage is now an extra load between live and neutral and wont trip an RCD.
This is potentially DANGEROUS. DONT do this with anyone else in the house, nor with pets around.
Touching the cooker could be FATAL.
Put some sort of barrier around the cooker in case you forget.
Connecting the earth terminal of the cooker to mains neutral is a lower risk than leaving it floating and almost certainly live via the leakage. Still NOT SAFE though, be very careful.

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Old 7th Apr 2018, 7:20 pm   #19
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Default Re: Creda Freestanding electric cooker. 1970s

Thanks everyone for your advice and suggestions. This is the first electric cooker I have owned and I appreciate the help so far.

Today I've run the top oven upto maximum temperature and this seems to have helped because all four hob rings will now work without tripping the rcd, when used alone. I think with a bit more use this will improve further to the point where they can be used together.

The only thing not working now is the bottom oven. Tomorrow I'm going to run the top oven again to try and heat up the unit and see if this helps!
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Old 7th Apr 2018, 7:40 pm   #20
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Default Re: Creda Freestanding electric cooker. 1970s

Quote:
Originally Posted by LukeG83 View Post
The only thing not working now is the bottom oven. Tomorrow I'm going to run the top oven again to try and heat up the unit and see if this helps!
Maybe this approach will work better if you turn the thing upside-down first.

(I think I'm joking, but, you know... )
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