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Topic Title: Surge protection devices and testing.
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Created On: 14 July 2011 12:44 PM
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 14 July 2011 12:44 PM
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postman

Posts: 56
Joined: 03 July 2011

Hi all.
Another 2391 question I am not sure about.
The question:

A 230V circuit containing a surge protection device is to be tested for insulation resistance and the device cannot be removed. State the.
a) Conductors between test is to be carried out.
b) test voltage applied.
c)minimum acceptable resistance.

Now I have looked at GN3 and also BS7671 page 158 notes under table 61.

The notes states:
"where surge protective devices cannot be removed for example fixed socket incorporating SPD the test voltage may be reduced to 250V but the insulation resistance will still have to be 1MOhm

What is confusing me is the conductors to be tested.
No where in the notes does it say which ones.
Is the test 250V DC between line and neutral, then between line and neutral and earth?
Is the test between joined line and neutral conductors and to earth only?

I am finding it hard to get a straight answer on this one within my books.
Any opinions would be welcome.
Thanks.
 14 July 2011 07:04 PM
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kj scott

Posts: 2144
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Originally posted by: postman

Hi all.

Another 2391 question I am not sure about.

The question:



A 230V circuit containing a surge protection device is to be tested for insulation resistance and the device cannot be removed. State the.

a) Conductors between test is to be carried out.

Page 39 of GN3

b) test voltage applied.

Note below table 61 BS 7671

c)minimum acceptable resistance.

Note below table 61 BS 7671


Now I have looked at GN3 and also BS7671 page 158 notes under table 61.



The notes states:

"where surge protective devices cannot be removed for example fixed socket incorporating SPD the test voltage may be reduced to 250V but the insulation resistance will still have to be 1MOhm



What is confusing me is the conductors to be tested.

No where in the notes does it say which ones.

Is the test 250V DC between line and neutral, then between line and neutral and earth?

Is the test between joined line and neutral conductors and to earth only?



I am finding it hard to get a straight answer on this one within my books.

Any opinions would be welcome.

Thanks.


-------------------------
http://www.niceic.biz
 14 July 2011 07:05 PM
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rocknroll

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Originally posted by: postman

Hi all.

Another 2391 question I am not sure about.

The question:

A 230V circuit containing a surge protection device is to be tested for insulation resistance and the device cannot be removed. State the.

a) Conductors between test is to be carried out.

b) test voltage applied.

c)minimum acceptable resistance.

Now I have looked at GN3 and also BS7671 page 158 notes under table 61.

The notes states:

"where surge protective devices cannot be removed for example fixed socket incorporating SPD the test voltage may be reduced to 250V but the insulation resistance will still have to be 1MOhm

What is confusing me is the conductors to be tested.

No where in the notes does it say which ones.

Is the test 250V DC between line and neutral, then between line and neutral and earth?

Is the test between joined line and neutral conductors and to earth only?

I am finding it hard to get a straight answer on this one within my books.

Any opinions would be welcome.

Thanks.


The SPD comes into play at voltages higher than 250V so readings could be somewhat iffy as a leakage current will flow through the SPD.

a. All conductors
b. 250v
c. 1 megohm

If the readings are suspect you need to disconnect the SPD and any associated electronic equipment and do the standard test.

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: 14 July 2011 at 07:16 PM by rocknroll
 14 July 2011 07:18 PM
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Avatar for kj scott.
kj scott

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a. All conductors

That not what GN3 recommends

b. 250v

250V d.c.

c. 1 megohm

Not less than 1MΩ.

-------------------------
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 14 July 2011 09:46 PM
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rocknroll

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Well in their hurry to copy a statement that has been in IEC 60364 and many standards around the world for a least twenty years:

Note: Where surge protective devices (SPDs) are likely to influence the test or be damaged, such equipment shall be disconnected before carrying out the insulation resistance test. Where it is not reasonably practicable to disconnect such equipment (e.g. in case of fixed socket-outlets incorporating and SPD), the test voltage for the particular circuit may be reduced to 250 V d.c., but the insulation resistance must have a value of at least 1 Mohm.

They have failed to recognise that people have been testing circuits and leads that contain SPD's and have been testing all conductors at 250V dc so there has to be an error.

The IET have signed up to this;

Where there is sensitive electronic equipment a test at 500V dc should be conducted with the L and N connected

or

All conductors should be 'soft tested' at a voltage of 250V dc and in both cases the minimum insulation resistance should be 1Mohm

Although it is possible that a circuit with an SPD will contain sensitive electronic equipment it is not the prime reason to use 250V dc it is highly likely the SPD will contain MOV's (metal oxide varistors) 3 in fact connected to L-N, N-E and L-E and they conduct around the 297/300 mark so a test of 500V would cause them to conduct and give a false reading, the test conducted at 250V would give a more accurate reading, so it does not make sense to joining L-N together for this test unless in my years out of the loop I am missing something, perhaps the IET who are trawling this forum for future staff could enlighten us.

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: 14 July 2011 at 09:57 PM by rocknroll
 14 July 2011 09:54 PM
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Avatar for postman.
postman

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Edit:
I wrote this post as you were writing yours rocknroll, I had not read your post while I was writing this.

BS 7671 page 158 note under table 61.

"where surge protective devices or other equipment are likely to influence the verification test or be damaged such equipment shall be disconnected before carrying out the insulation resistance test. where this is not practical the test voltage for the particular circuit may be reduced to 250V DC but the insulation resistance value shall be at least 1 MOhm.2

Also same page REG 612.3.1
"where appropriate during the measurement line and neutral conductors may be connected together"

Also REG 612.3.3
"Where the circuit included electronic devices which are likely to influence the results or be damaged only a measurement between the live conductors connected together and the earthing arrangement shall be made"

Now couple this with the examiners report dated October 2010.
He states.
"there were very few candidates who understood the requirements for insulation resistance testing a circuit containing a surge protective device which cannot be removed. Most linked line and neutral and tested at 500V".
Unfortunately he does not state the correct way of testing them himself.

So will a test at 250V DC across line and neutral be OK for a surge protective device, I mean if it is operating correctly it should give a good reading at that voltage so it should not influence the results and
It would not be damaged at 250V.

So I am still confused, do I link line and neutral and test between them and earth at 250V DC or do I test between line and neutral separately and also line and neutral to earth?
The one thing I know is wrong is I cannot link line and neutral together and test at 500V.

I'm still not sure.
I would love to know.
 14 July 2011 09:57 PM
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kj scott

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I believe that the OP was looking for the 2391 model answers, which is not necessarily the same thing.
That aside, GN3 does state that the live conductors should be joined together, for the test between live conductors and earth in such circumstances, and BS 7671 recommends the test voltage be 250V d.c. when SPD's can not be disconnected.

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 14 July 2011 10:02 PM
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postman

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From what I have read it sounds like a good idea to test between all conductors as this would also test the surge protective devices were working correctly but as has been said this is a question for a 2391 exam and the answer needs to satisfy them.
 14 July 2011 10:10 PM
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rocknroll

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this would also test the surge protective devices


Thats a difficult one, as they tend to default open circuit so you have no indication, if by a miracle they defaulted closed circuit you would already have problems like RCD's or breakers tripping.

There are SPD testers available but the price might put you off a bit, more suited for the Golden Wonder boys who love to collect every gadget under the sun.

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: 14 July 2011 at 10:21 PM by rocknroll
 14 July 2011 10:39 PM
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postman

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rocknroll I am not trying to suck up to you or anything but I just read a thread about you talking about a website you once had.

I have to say that my own 2391 course involved my tutor fumbling his notes, forgetting what he was saying, bringing the wrong notes, not turning up sometimes, and generally just making us do past papers without any teaching input at all.

He could not even get TN-C-S or TN -S right, the one thing he knew how to do was draw an earth fault loop path for a TT system which he spent ages showing us.

All he really wanted to do was talk about his job at the airport which he was now retired from and the holidays he was looking forward to paid for by his generous pension.

I can safely say he was the worst tutor I have ever met in my entire life.

We spent one evening in the test room looking at the rig and he said.
"Don't worry lads everyone passes the practical"
Or words to that effect.

The biggest joke, apart from the joke on us for paying the money, was the guys showing the tutor how to use the test equipment a Megger 1552.
I mean I've never seen anything like it.

So I apologise if these questions seems a bit obvious but as you can see I was taught basically nothing at my twelve week night school course.

I am determined to to as best I can in this exam despite all this and the comments here are very much appreciated.

Rant over.

Edited: 14 July 2011 at 10:56 PM by postman
 14 July 2011 11:44 PM
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rocknroll

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All he really wanted to do was talk about his job at the airport which he was now retired from and the holidays he was looking forward to paid for by his generous pension.


You have obviously hit the nail on the head there, all too often a student walks into a classroom ready to learn, but the teaching system is full of the retired, redundant and wannabe tutors who are just biding their time and collecting their pocket money because they have no future, as soon as you hear the common statement "In my day, yada yada yada!" as a student you know damn well this is going to be about one person and a f****** history lesson and not about your present and future needs, so you switch off realising at the end of the day there is no-one there to help you, what we need is some dynamic young tutors who's future and development is on the same plane as yours.

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!"
-------------------------
 14 July 2011 11:56 PM
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potential

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On the subject of tutors, I recall a tutor on my ONC course was discussing the construction of capacitors.
He described the construction of a flat capacitor then went on to say they were often rolled up to save space.
I raised the point that the rolling also doubled the capacitance because the process utilised both sides of the plates which effectively doubled the plate areas.

He wouldn't have it.
He insisted that the construction of a rolled capacitor in cross section was:
positive plate - dielectric - negative plate - "insulation" - positive plate etc.
He couldn't get into his head that the "insulation" (as he called it) also acted as a dielectric .
The argument lasted the whole lesson.
I never took him seriously after that.
 15 July 2011 02:09 PM
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Avatar for postman.
postman

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Just one question to clarify on SPD.
Do they offer protection against surges across all three conductors.
Have they got equipment to protect against surges not only across line and neutral but also across line to earth..
I guess they would.
Surely they would not have equipment to protect against surges from neutral to earth.
As I am not sure how they actually work inside and so I am a little at a loss.
Thanks.
 15 July 2011 02:31 PM
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rocknroll

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The majority of SPD's in use are MOV's (Metal Oxide Varistors) and the voltage rating of the MOV is around twice the supply voltage and the clamping voltage is approx 297/300V, the MOV under a normal state has a high resistance path, when a surge occurs the MOV switches to a low resistance path and the surge voltage passes to earth by-passing the connected load.

If you draw out the connections L-N, N-E and L-E from my above post you will see the path the surge will follow when the MOV's are in low resistance state.

In the more quality SPD's there is often an inductor in series with the MOV to slow down the steady rise in current.

Am I behaving myself.

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!"
-------------------------
 15 July 2011 03:18 PM
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postman

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Originally posted by: rocknroll
If you draw out the connections L-N, N-E and L-E from my above post you will see the path the surge will follow when the MOV's are in low resistance state.regards


So a surge from Line would go line to earth and also separately from line to neutral and then neutral to earth.
Sort of giving us two routes to go in case one route failed.

Is there any other reason we would get a surge in the neutral apart from the fact the line surge protector had dumped it there in the first place?

With the cheap ones do they run the risk of blowing up when a large current passes due to the surge?

They sound like pretty good pieces of kit but I thought we had a nice stable electricity supply that was fairly free of these voltage spikes.
Are they really necessary?
I've never had any problems with surges of the electrical kind.

Thanks.

Edited: 15 July 2011 at 03:46 PM by postman
 15 July 2011 05:31 PM
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jcm256

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I suppose by stating the "device cannot be removed" lets you know that they don't mean a SPD fitted in a consumer unit whereas to do a insulation test (all in test) may need to be removed.

http://www.hagergroup.ae/energ...ective-devices/983.htm
 15 July 2011 10:54 PM
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spinlondon

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Must admit that some of the replies appear to be slightly confusing.
To answer the OP's question as clearly as posible, the test should be conducted between each individual conductor, at 250V d.c.
If you read BS7671 you will note that the reference to method 2 where L&N are linked and tested to earth, is in the next paragraph to the one refering to SPDs.
If it was intended that test method 2 is to be used with SPDs, the paragraphs would be the other way round.
 15 July 2011 11:13 PM
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kj scott

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Spin methods of testing are not given in BS 7671, but are outlined in GN3 which clearly states,
''Test 2- Insulation resistance to earth
Single-phase
Test between the line and neutral conductors connected together and earth at the appropriate distribution board, or for circuits/equipment not vulnerable to insulation resistance testing, line and neutral separately to earth.''

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 16 July 2011 12:13 AM
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spinlondon

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They are in my copies.
"Regulation 612.3.3 Where the circuit includes electronic devices which are likely to influence the results or be damaged, only a measurement between the live conductors connected together and the earthing arrangement shall be made."
 16 July 2011 12:18 AM
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kj scott

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BS 7671 does not refer to method 2, 612.3.3 is the same as the information in GN3, would you not agree?

-------------------------
http://www.niceic.biz
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