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Topic Title: Heating Elements with Low Voltage Transformer
Topic Summary: Using the earth as a return wire on a heating element on 1-ph system.
Created On: 12 September 2017 09:44 AM
Status: Read Only
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 12 September 2017 09:44 AM
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lesspiteri

Posts: 9
Joined: 14 March 2010

Hi

I have got myself confused over an AC circuit, where it was explained to me that a heating element can use the earth wire as the return (neutral) from a low voltage transformer secondary output (12VAC).

Surely this is not within regulations, I have always been taught that the secondary of the isolating transformer should have the 0V (the neutral) connection to EARTH only.

Therefore two wires go to the heating element (hot and neutral) If the second wire goes to earth then on a fault condition on that circuit the metal casing of the heating device could become live - albeit at only 12V.

Many thanks for any help.
 12 September 2017 10:16 AM
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Ricicle

Posts: 894
Joined: 23 October 2006

An isolating transformer will have no output connection to earth so you will only have two wires to connect to the element from the secondary of the Tx. Are you in the UK?

-------------------------
Empty barrels make the most noise.
 12 September 2017 01:16 PM
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mapj1

Posts: 9557
Joined: 22 July 2004

On the secondary side of the transformer you can connect one end or even the middle of the winding to earth. The only bit of the earth path that is carrying any current is the small loop internal to the device - you are not actually using the supply earth as part of the current path, it just happens to connect to one side of it.

-------------------------
regards Mike
 13 September 2017 11:21 AM
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lesspiteri

Posts: 9
Joined: 14 March 2010

Thanks for the response, yes I am the UK.

I think I understand both posts however I never explained the actual problem properly.

The heating element only has one connection for live and the second connection is connected against the outer casing which is earthed. I can understand this will heat up (the return path through to earth) however I thought this must be dangerous as I think regulations state you cannot use earth wire as a neutral. This means I would be using the the supply earth as the return path albeit through the casing then through an earth wire.

It was explained to me it was the same as a car where the return is the chassis. That sort of makes sense but this is not DC and if I am indeed correct where you cannot use the earth as a return then it should not matter if the voltage is only 12Vac - it is still not a permitted method?

Sometimes you think you have a good understanding then something like this happens. Ha
 13 September 2017 01:55 PM
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ectophile

Posts: 747
Joined: 17 September 2001

This sounds like PELV. Presumably there is a return wire from the heater to the transformer, otherwise It wouldn't work. In this case, it's just a return wire that happens to be earthed.

-------------------------
S P Barker BSc PhD IEng MIET
 14 September 2017 08:17 AM
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lesspiteri

Posts: 9
Joined: 14 March 2010

Thanks so much that has really helped me.

So I have been reading 7671, and the bit that is still worrying me is that the 2nd heater connection is to the heater chassis which is then earthed to the return on the transformer. Would this still comply with PELV?
 14 September 2017 05:38 PM
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mapj1

Posts: 9557
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yes - you are not using the CPC to carry load current are you?, you are using the metal of the equipment as the return to the Xformer, and it has one, and only one, connection to mains earth ?

-------------------------
regards Mike
 15 September 2017 12:50 PM
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lesspiteri

Posts: 9
Joined: 14 March 2010

The transformer has two connections from 0V (on my drawings - not actually wired).
1. To the enclosure earth stud where the mains incoming earth is also terminated.
2. The second connection comes from the chassis of the heater, an earth wire (cpc) is bolted on and returns to the transformer (0V).

I had presumed that number 2 connection ( cpc wire bolted to chassis was required to give a "good" return to the transformer, is this an incorrect presumption?

So I am not using the cpc to carry load but it is the return to the transformer which is why I got so confused in the first place as I had thought the cpc could never be used as a neutral however since then PELV has been pointed out by ectophile (above) which gave me a slightly better understanding.
 15 September 2017 04:49 PM
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westfield6

Posts: 210
Joined: 12 October 2007

12 volts is NOT low voltage. 240 volts is low voltage. 12 volts is extra low voltage. You need to use the right terms.
 15 September 2017 05:31 PM
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Alcomax

Posts: 245
Joined: 12 November 2009

Originally posted by: westfield6

12 volts is NOT low voltage. 240 volts is low voltage. 12 volts is extra low voltage. You need to use the right terms.

Unfortunately the "low voltage" thing is everywhere , like "ring main". Blame wholesalers and the sheds , refer to anything not 230 v as " low voltage"
 15 September 2017 06:04 PM
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geoffsd

Posts: 1759
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Originally posted by: westfield6
12 volts is NOT low voltage.

Yes it is. ELV is part of LV.

Originally posted by: Alcomax
Unfortunately the "low voltage" thing is everywhere , like "ring main". Blame wholesalers and the sheds , refer to anything not 230 v as " low voltage"

It is.

Even Extra Low Voltage is a meaningless term as it could mean 3V, 6V, 12V, 24V, 48V and Low Voltage can be that or anything else in between and up to a 1000V.

The only way to be certain is to quote the actual voltage - which you will have to do anyway.

Edited my mistake thanks to Alcomax

Edited: 15 September 2017 at 07:15 PM by geoffsd
 15 September 2017 07:06 PM
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Alcomax

Posts: 245
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Originally posted by: geoffsd

Originally posted by: westfield6

12 volts is NOT low voltage.


Yes it is. ELV is part of LV.



Originally posted by: Alcomax

Unfortunately the "low voltage" thing is everywhere , like "ring main". Blame wholesalers and the sheds , refer to anything not 230 v as " low voltage"


It is.



Even Extra Low Voltage is a meaningless term as it could mean 3V, 6V, 12V, 24V, 48V or anything else in between and up to a 1000V.



The only way to be certain is to quote the actual voltage - which you will have to do anyway.


Edit to actually type a reply!

Eh? We are talking "supply system voltage " as in uk?

Please elaborate on the ELV up to 1000v?
 15 September 2017 07:16 PM
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geoffsd

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Oops, sorry. My mistake. Thank you.

I have corrected it.
 15 September 2017 07:28 PM
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Alcomax

Posts: 245
Joined: 12 November 2009

British Standard BS 7671:2008 defines supply system low voltage as:

50 - 1000 V AC or 120 - 1500 V ripple-free DC between conductors;
50 - 600 V AC or 120 - 900 V ripple-free DC between conductors and Earth.

(BS 7671:2008) define an ELV device or circuit as one in which the electrical potential between conductor or electrical conductor and earth (ground) does not exceed 50 V a.c. or 120 V d.c. (ripple free).



Is ELV part of LV?


I can see a long thread coming on?
 15 September 2017 07:51 PM
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geoffsd

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 15 September 2017 08:08 PM
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Alcomax

Posts: 245
Joined: 12 November 2009

Originally posted by: geoffsd

Originally posted by: Alcomax



Is ELV part of LV?




According to the IEC



">"><br ">&l......./ie...f=151-15-03


]http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=151-15-03
<br [/L]


http://www.electropedia.org/ie...nform&ievref=826-12-30


Geoff, with reference to second link :

But ELV is in Band 1
LV is in Band II

This is a cryptic quiz?

Edited to add :

So the cryptic bit is "distribution", as " A/C distribution of electric power" ......that is subject to context. I am sure Denmans did not go through that thought process when deciding to not use the term Extra Low Voltage in their monthly comic.

Other edit : Changed to smiley, hit red angry by mistake...the button is close to it!

Edited: 15 September 2017 at 08:45 PM by Alcomax
 15 September 2017 08:37 PM
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gkenyon

Posts: 4954
Joined: 06 May 2002

Originally posted by: Alcomax

Originally posted by: geoffsd



Originally posted by: AlcomaxBS 7671 defines ELV and LV and HV under "Voltage, nominal".

THe definition for LV begins "Exceeding extra-low voltage, but not exceeding".

Therefore for BS 7671, ELV is NOT LV.

In terms of the IEV references, the definitions in 151-15-03 and 151-15-04 are for electrical and magnetic devices, not installations. BS 7671 does not conflict, just qualifies.







Is ELV part of LV?








According to the IEC







"><br ">http://...../ie...f=151-15-03







http://www.electropedia.org/ie...nform&ievref=826-12-30




Geoff, with reference to second link :



But ELV is in Band 1

LV is in Band II



This is a cryptic quiz?



Edited to add :



So the cryptic bit is "distribution", as " A/C distribution of electric power" ......that is subject to context. I am sure Denmans did not go through that thought process when deciding to not use the term Extra Low Voltage in their monthly comic.


-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 15 September 2017 08:53 PM
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Alcomax

Posts: 245
Joined: 12 November 2009

Sorry you inherited the angry emoticon on your post Mr Kenyon.

Now I have changed mine it is sort of out of context!
 18 September 2017 10:23 AM
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lesspiteri

Posts: 9
Joined: 14 March 2010

Thanks for pointing that out "westfield6" - apologies the title would have been better reading "Extra Low Voltage Transformer.
 18 September 2017 10:26 AM
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lesspiteri

Posts: 9
Joined: 14 March 2010

Regarding PELV, the thread had stalled at this point if there were any further comments?

The transformer has two connections from 0V (on my drawings - not actually wired).
1. To the enclosure earth stud where the mains incoming earth is also terminated.
2. The second connection comes from the chassis of the heater, an earth wire (cpc) is bolted on and returns to the transformer (0V).

I had presumed that number 2 connection ( cpc wire bolted to chassis was required to give a "good" return to the transformer, is this an incorrect presumption?

So I am not using the cpc to carry load but it is the return to the transformer which is why I got so confused in the first place as I had thought the cpc could never be used as a neutral however since then PELV has been pointed out by ectophile (above) which gave me a slightly better understanding.
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