IET logo
 
IET
Decrease font size
Increase font size
Topic Title: SECTION 705 OF THE BGB
Topic Summary: A CONUNDRUM
Created On: 05 August 2014 11:26 PM
Status: Post and Reply
Linear : Threading : Single : Branch
<< 1 2 3 4 5 6 7 Previous Next Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 07 August 2014 05:27 AM
User is offline View Users Profile Print this message


Avatar for GeoffBlackwell.
GeoffBlackwell

Posts: 3588
Joined: 18 January 2003

John - the answer to this maybe in your own hands

My information (from the horses mouth {ouch} ) is that 705 was put together by just three people - a German, an Italian and an Englishman.

Said Englishman is, I believe, known to you, AJ, Zs and many others. Go and bang on his door and ask for a solution ,

On a more serious note the problem is most likely caused by a low pass supply line filter, similar in operation to the types used on IT equipment, but with a higher designed leakage current to earth. In the good old days you could have instigated a prosecution under the Electricity Supply Regulations (1936 if memory serves). These prohibited leakage greater than 1/10000 part of the maximum current supplied - but that was removed to allow for the emerging IT industry.

So what is the solution - well if this equipment is essential - there is none other than some combination of the ground fault sensing mccb (as offered by OMS) and some form of protective bonding and, as BOD has indicated, someone is going to have to stick their neck out and confirm that it provides the required degree of protection.

I had a similar case (that I have probably posted before). It involved a certain secretive organisation based in Cheltenham - they use very large filters to suppress any signals that might otherwise leak on to the supply network. I offered them various solutions including the use of isolating transformers. They listened but I think they were just humouring me- finally they said "don't concern yourself Mr Blackwell - if we do kill anyone we won't tell you .

Regards

Geoff Blackwell
 07 August 2014 10:01 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

"don't concern yourself Mr Blackwell - if we do kill anyone we won't tell you .


or anyone else in fact

I've had similar problems at other "establishments" where the "filters" are top of the tree in terms of requirements - everything else is a secondary consideration

Regards

OMS

-------------------------
Failure is always an option
 07 August 2014 10:08 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 2019
Joined: 21 November 2008

I think you two are going soft.

You should have threatened them with the omni power of BS7671! That would have put the willies up them!
 07 August 2014 11:19 PM
User is offline View Users Profile Print this message


Avatar for sparkingchip.
sparkingchip

Posts: 6326
Joined: 18 January 2003

A question for you mathematicians, how much power does a 300mA fault dissipate?
 08 August 2014 12:17 AM
User is offline View Users Profile Print this message


Avatar for mapj1.
mapj1

Posts: 3419
Joined: 22 July 2004

Depends ! - if it is resistive , in the form for example of ~750 ohms from one phase to ground - circa 80 watts,
or purely reactive, in which case the correct answer is 'nil'. In the reactive case, the current is most when there is no volts present and vice versa, and the integral of I *V over a whole cycle spends as much time above as below the line and adds up to a nice round zero, at least in my typeface..
(One way to think of this for a capacitor is that current rushes into a flat capacitor even with low voltages, but when it is full, at the top of the sine wave, as the line voltage drops below that stored on the capacitor, the current starts to come out, and the capacitor empties backwards into the line, and you get all, or in reality 'most' of your energy back. A similar magnetic analogy applies to coils, but the mental picture is less straight forward as we don't normally consider inductors to 'charge', and things like saturation and hysteresis muddy the waters..)


Of course in a real situation there is always some resistance and therefore some heat loss, even if its just the Zs in series with a 'pure' capacitor, and actually at higher freqs a pure capacitor is harder to find

But if its 700j ohms in series with even a very high 10 ohms of loss, then the power converted to heat that has to be sweated off, is much more bearable about one watt.

-------------------------
regards Mike
 08 August 2014 01:59 AM
User is offline View Users Profile Print this message



Jaymack

Posts: 4785
Joined: 07 April 2004

Originally posted by: sparkingchip
A question for you mathematicians, how much power does a 300mA fault dissipate?

Rather - what is the incendiary effect in dynamic J/s of the resistive element of this fault path, in consideration of the flammability of the immediate surroundings. (The length of string is the unknown).

Regards
 13 August 2014 05:38 PM
User is offline View Users Profile Print this message



weirdbeard

Posts: 1652
Joined: 26 September 2011

Originally posted by: John Peckham

As I am sure you all know 705.411.1 and 705.422.7 requires all circuits on a farm to be 300mA RCD protected for ADS and fire protection regardless of the earthing system (it's TNS).



On a farm I EICRed the electrical contractor is doing all the required remedial works to bring the farm up to current standards.




Hi John, how olds the installation, is it well maintained or generally shabby, and what code did you give on the EICR for this?
 13 August 2014 09:08 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3596
Joined: 22 November 2007

Sorry guys and girls, been away from the forum for a few days.

We had an ABB drive doing almost exactly the same except it was driving a large motor on a grain conveyor.

After discussing the matter with ABB technical, there are apparently two screws on the side of an ABB drive. One it seems provides an earth reference for an EMC filter, the other an earth reference for something else.

Removing the "something else" screw on their advice sorted the problem.

Stu
 13 August 2014 10:08 PM
User is offline View Users Profile Print this message


Avatar for rocknroll.
rocknroll

Posts: 8905
Joined: 03 October 2005

Originally posted by: sparkingchip

A question for you mathematicians, how much power does a 300mA fault dissipate?


Just for info;

The IEC has specified an upper limit of 300mA for RCD's intended for fire protection. This is based on a power level of about 60 watts which is considered to be sufficient to cause an electric fire, this level is actually exceeded at 230V for a 300mA residual current. Any RCD with a rated residual operating current not exceeding 300mA will meet this requirement.

Maths
P = VI
P = .3 x 230
P = 69 watts

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 13 August 2014 at 10:23 PM by rocknroll
 14 August 2014 01:00 PM
User is offline View Users Profile Print this message


Avatar for Parsley.
Parsley

Posts: 1090
Joined: 04 November 2004

Have you managed to resolve the issue John?

Regards
 14 August 2014 01:18 PM
User is offline View Users Profile Print this message


Avatar for sparkingchip.
sparkingchip

Posts: 6326
Joined: 18 January 2003

I'd come up with 69 watts myself before posting and considered that you could potentially run a 60 watt lamp connected live/ earth without tripping the RCD and with the heat produced start a fire if for example it was in a inspection lamp in the workshop left in a inappropriate position. So on the face of it 300mA is quite generous and doesn't provide close protection against fire.

Some how though I figured there was a more involved answer!

I was actually wondering if you took the offending circuit off a shared RCD that you could then have a dedicated RCD for the equipment and having removed background leakage elsewhere in the installation reduce the rating of the dedicated RCD to 100mA, so although there would be multiple RCD's there would be a better level of protection?

Andy
 14 August 2014 01:21 PM
User is offline View Users Profile Print this message


Avatar for Parsley.
Parsley

Posts: 1090
Joined: 04 November 2004

http://webarchive.nationalarch...tynetwork/pdf/rcd.pdf

I seem to remember someone asking about RCDs and fire a while ago it might have been Zs and someone posted the link to the document above.

Regards
 14 August 2014 01:42 PM
User is offline View Users Profile Print this message



davebp

Posts: 116
Joined: 16 April 2009

Probably not very helpful in this instance, but for reference - I've had similar problems before with smaller drives; the advice was to relocate the drive as close to the motors as possible, as this significantly reduces the leakage current produced. I'm not clever enough to be able to explain why though!

Dave
 15 August 2014 06:11 PM
User is offline View Users Profile Print this message



weirdbeard

Posts: 1652
Joined: 26 September 2011

Originally posted by: weirdbeard



Hi John, how olds the installation, is it well maintained or generally shabby, and what code did you give on the EICR for this?


Must admit I'm still a wee bit curious as to why the contractors felt the need to be retro-fitting rcds to the existing installation?
 15 August 2014 06:59 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

Originally posted by: weirdbeard

Originally posted by: weirdbeard

Hi John, how olds the installation, is it well maintained or generally shabby, and what code did you give on the EICR for this?


Must admit I'm still a wee bit curious as to why the contractors felt the need to be retro-fitting rcds to the existing installation?


Presumably to make the installation compliant with Section 705 of BS 7671

Regards

OMS

-------------------------
Failure is always an option
 15 August 2014 07:16 PM
User is offline View Users Profile Print this message



weirdbeard

Posts: 1652
Joined: 26 September 2011

Originally posted by: OMS

Originally posted by: weirdbeard


Originally posted by: weirdbeard



Hi John, how olds the installation, is it well maintained or generally shabby, and what code did you give on the EICR for this?




Must admit I'm still a wee bit curious as to why the contractors felt the need to be retro-fitting rcds to the existing installation?




Presumably to make the installation compliant with Section 705 of BS 7671



Hi OMS, thanks for your reply, what code would you say johnP gave in relation to this RCD deficiency for the existing installations EICR?
 15 August 2014 07:46 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

Hi OMS, thanks for your reply, what code would you say johnP gave in relation to this RCD deficiency for the existing installations EICR?


Hang on - I'll go and check my crystal ball !!

No idea - but either a C1 or a C2 based on the presumption that it was unsatisfactory and thus a contractor was engaged to rectify.

I can only presume the defect was listed against 705.411.1 (iii) and 705.422.7 - how the contractor chose to address that is presumably down to the contractor - and probably his inability to recognise the design effort required to deploy commercial off the shelf (COTS) solutions when protecting supplies to VSD controlled motors - so I guess that's what's lead to the use of Type A or Type AC RCD's rather than Tye B or super immunised (Type Si) RCD's or to MCCB's with enabled ground fault protection.

Alternatively, the EICR listed both the defect and the solution and the contractor has just implemented the recommendations

I honestly don't know what the situation is - I only have the information you do, based on what's posted.

Regards

OMS

-------------------------
Failure is always an option
 15 August 2014 08:06 PM
User is offline View Users Profile Print this message



weirdbeard

Posts: 1652
Joined: 26 September 2011

Originally posted by: OMS

Hi OMS, thanks for your reply, what code would you say johnP gave in relation to this RCD deficiency for the existing installations EICR?




Hang on - I'll go and check my crystal ball !!



No idea - but either a C1 or a C2 based on the presumption that it was unsatisfactory and thus a contractor was engaged to rectify.






Hi OMS, could also be a C3? John must have missed my Q - just wondered what you thought of it
 15 August 2014 08:17 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

Hi OMS, could also be a C3? John must have missed my Q - just wondered what you thought of it


It could be a C3, yes - but wouldn't you think that unlikley given that there is a very direct and specific requrement in BS 7671 for 300mA RCD protection which would render a C3 as inappropriate

I guess you'll need to get a response to the actual code from JP himself - I'm only guessing

Regards

OMS

-------------------------
Failure is always an option
 15 August 2014 08:20 PM
User is offline View Users Profile Print this message



perspicacious

Posts: 7267
Joined: 18 April 2006

"The IEC has specified an upper limit of 300mA for RCD's intended for fire protection. This is based on a power level of about 60 watts which is considered to be sufficient to cause an electric fire, this level is actually exceeded at 230V for a 300mA residual current. Any RCD with a rated residual operating current not exceeding 300mA will meet this requirement."

This 60 W may have its roots in C.12 of the 1966 14th....

Regards

BOD
IET » Wiring and the regulations » SECTION 705 OF THE BGB

<< 1 2 3 4 5 6 7 Previous Next Last unread
Topic Tools Topic Tools
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.