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Topic Title: Minimum bending radius in T&E cable
Topic Summary: Why the same on major and minor axes?
Created On: 27 March 2007 02:09 PM
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 27 March 2007 02:09 PM
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khales

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Joined: 16 January 2002

According to the IEE On Site Guide (Table 4E), NICEIC Technical Handbook, etc., the minimum bending radius for T&E (flat) cable is based on the diameter of the major axis.
I've yet to meet an electrician (not including myself, of course), who doesn't allow themselves a smaller radius when curving T&E cable on the minor axis.
This is causing me much agro on inspections; because people don't like being told their bends are too tight. They ask why, and say things like: 'I wouldn't be able to get the T&E around the corners in my surface trunking, etc.'

Experience tells me I might just as well have all my PIRs and 3-signature EICs pre-printed with a Code-2 for this aspect. However, I can't really think of any technical reason as to why bends on the minor axis of T&E couldn't be allowed to be tighter than on the major radius; e.g. based on the same multiple of diameter.

Comments please, and how strictly do others here enforce the minimum bending radii for T&E cables on themselves, and on third parties?

-------------------------
khales

Edited: 28 March 2007 at 08:36 AM by khales
 27 March 2007 07:10 PM
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normcall

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I still fold mine whenever possible - throwback to 7/029 when fixed cables were much more pliable.
Biggest problem is cold insulation which limits the 'bend'.

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Norman
 27 March 2007 08:09 PM
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beaver74

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. However, I can't really think of any technical reason as to why bends on the minor radius of T&E couldn't be allowed to be tighter than on the major radius; e.g. based on the same multiple of diameter.



?


i have heard of t&e cables that have split at the internal pvc due to presure applied throung tight bend ect
 27 March 2007 09:41 PM
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normcall

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More prone when done cold. This new fangled brown and blue stuff is worse than the Red/Black variety I find.
Trouble is I can remember when there was winter and summer compounds for cable.

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Norman
 28 March 2007 08:35 AM
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khales

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I've looked into this a bit more. BS7671 522-08-03 simply requires avoiding damage. It's the interpretation of this requirement in the On-Site Guide and other publications that I'm querying. It doesn't make sense to me that the bend radius for a flat cable has to be the same regardless of the direction of the bend. I'm inclined to allow a smaller bend radius on the minor axis; but does anyone know why publications say the minimum bend radius for flat cables, regardless of bend direction, must be based only on the overall diameter measured on the major axis? I don't have the BS standards for T&E cables to hand; but perhaps it comes from there? Can anyone say definitively?

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khales
 28 March 2007 12:58 PM
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OMS

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Not sure you are going to get a definitive answer to this one - going from memory here, the product standard for flat twin and earth (BS 6004) doesn't cover installation methods.

This leaves BS 7540 part 1 and I think part 3, as T&E is not a harmonized cable type - the latter doesnt cover bending radii - the former is very generic and doesn't draw the distinction between major and minor axis in Tables of bending radii. Manufacturers may of course have a view or requirement.

Perhaps the conclusion is that OSG, GN 1 etc base the statement on "custom & practice" rather than any hard fact. Essentially, it probably comes back to reg 522 - 08 - 03 and a healthy dollop of engineering judgement - sorry

Regards

OMS

-------------------------
Failure is always an option
 28 March 2007 02:20 PM
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davezawadi

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I'm not surprised people are unhappy with a code for this "defect"! The guidance given in the on-site guide is a suggestion, not a regulation. 522-08-03 says no damage, meaning the conductor insulation and overall mechanical protection. A T&E cable inside another means of mechanical protection (mini trunking) can have as much or as little damage to the sheath as you like, in fact no sheath at all! As OMS says it is a matter of judgment, but as mini trunking has been fairly satisfactory for a long time with T&E contents, you should consider whether your coding is reasonable. Could you really defend your decision, against long experience from an expert witness? How many cases where a cable has failed due to insufficient bend radius have you seen? I have seen many where T&E has been bent around both inside and outside sharp corners with a hammer, yet the only splits which I remember were due to installation in very cold conditions! Try it with a pair of pliers and a bit of cable, the cores will fracture before the insulation if you bend it backwards and forwards in the same place.

A code 2 means there is a problem which is likely to lead to danger in the future (My words).
So you should not try to enforce this at all, it will just bring disrepute.

A new code 5 is being introduced - Not to NICEIC regulations, you could try that!

David

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David
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 28 March 2007 06:28 PM
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GeoffBlackwell

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A new code 5 is being introduced - Not to NICEIC regulations, you could try that!

They can't use that I own the copy rights to code 5, I have been using it for years .

Action codes for comments on the electrical installation
0 corrected during inspection - no action required

1 requires urgent attention - an immediate danger exists

2 requires improvement - safety is impaired

3 requires further investigation - no conclusion could be reached within the terms of this inspection, a further investigation is required to establish whether this aspect of the installation is safe

4 does not comply with BS 7671 - could be improved but is not necessarily unsafe

5 an improvement is recommended - an optional improvement is recommended as a matter of good practice

This allows me to vent any prejudices I may have without making them must be corrected defects.


Regards

Geoff Blackwell
 28 March 2007 06:44 PM
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OMS

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Ahh - this is where subjective opinion and personal preference start to creep into PIR's - amazing how many code 2 defects and code 3 investigations can "evaporate" when challenged

Perhaps the "code 5 simply" allows a PIR to become a condition report - not such a bad thing for your average client, many of whom think that's what they were paying for anyway

Regards

OMS

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 28 March 2007 07:35 PM
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khales

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Originally posted by: davezawadi...Could you really defend your decision, against long experience from an expert witness?...

To my mind that thinking is back-to-front. We all have to hand authoritative guidance and advice on this matter in the published forms of the On-Site Guide and the NICEIC Technical Manual. <B29-1> What I'm looking for is some justification for going (as you suggest) against that advice, so as to defend against the hypothetical expert witness who might feel inclined to point out that there is clear published guidance on the matter which should not have been ignored.

If the published guidance on minimum bending radii is unnecessarily strict, then I would prefer it was amended, rather than ignored. As it is; because I inspect to BS7671, not to NICEIC interpretation, then I do allow a tighter radius on the minor axis; but I draw the line at radii less than the tabulated recommendations.

-------------------------
khales

Edited: 28 March 2007 at 07:46 PM by khales
 28 March 2007 08:13 PM
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davezawadi

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Hello Geoff!

Wonderful to have both you and OMS agreeing!
I do have a prejudice against these code 2 things which have very little risk of danger. I also know that khales wants evidence to back this up. The trouble is that people write this stuff in books without sufficient experience and thought. If bending the T&E does no damage why is it a problem? Bending a bit of 70mm2 SWA through a right angle in an inch does, but not what you might expect strangely. I will leave proving this as an excercise for the student but you will need a powerful machine to do it! As I said, if bending the cable through a right angle at the smallest radius you can many times, just breaks the conductors what is the problem?

I do know what the objection to this view may be, it is that the PVC sheath is probably extended beyond its yeild point in theory. But because it is not anchoured to the conductor insulation and can move over some distance the movement is entirely elastic spread down the cable, so no actual damage. In fact the distance over which the sheath stretches is probably proportional to the inverse bend radius. Someone may enjoy checking up on this but not me! But it does mean that we do not observe a long term damage effect! Try cooling a piece of cable in a deep freeze for a few hours, then bend it at right angles as above. The PVC is now below its glass transition temperature and so is brittle. It will crack and chunks may even fly off - exactly the effect that cable bending radius tables may attempt to prevent! However once bent, the cooled it does not happen.

There is another side to this and that is at high temperatures, such a under overload. Say the conductors are at 70 degrees, then the PVC may creep, that is move to reduce the stress caused by the bending. PVC does this but not much below the melting point unless the interval is very long. It is this which controls the cable maximum operating temperature rather than the melting point.

All this may be interesting but does not say that the bend radius is a problem. The evidence in my view is that bend radius is not a significant factor in T&E installation. I would be pleased to hear any evidence in either direction.

Regards
David

-------------------------
David
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 28 March 2007 09:34 PM
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GeoffBlackwell

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Khales
I've looked into this a bit more. BS7671 522-08-03 simply requires avoiding damage. It's the interpretation of this requirement in the On-Site Guide and other publications that I'm querying.

So you accept that this specific bending radius requirement is not mentioned in BS 7671. Lets now look at the status of the OSG, GN1 to 7 (now 8 because a new one is being published) and any comments from other bodies such as my former employer the NICEIC.

Well quite simply all of these other publications are just the opinions of the bodies that produce them. None of whom "write the regulations". This includes the IET as they are not the sole authors of BS 7671, but they are the sole authors of the OSG and the GNs.

So when you are carrying out a PIR to BS 7671 any defect codes you apply should relate to departures from the requirements of BS 7671 and not interpretations that other bodies put on these requirements.

The regulation simply states that the bending radius shall be such that the conductors and cables do not suffer damage. So you need to establish if damage has occurred. If you cannot do this by your own efforts you could consult the manufacturer of the cable. If you are to use a code 2 (requires improvement - in BS 7671 or more explicitly - Safety is impaired, in my system) you need to show that the 'damage' is such that corrective action is essential. If you do not want extensive remedial work to be carried out do not use code 2 because this is a 'must correct' assessment.


Regards

Geoff Blackwell
 29 March 2007 12:05 AM
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press

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Just thought I would look up the specification and see what it said. I should have known better!!!
BS6004 for twin and earth doesn't say anything about bend radii but refers to BS7540-1 ' Electric cables - Guide to use for cables with rated voltage not exceeding 450/750 volts'
Section 5.6.3 Bending states ' the internal bending radii for different types of cable under normal conditions should not exceed values in table 2' then goes on'..... the bending radii recommended are for ambient temperatures of (20+-10) degrees C.
Table 2 states, Cables diameter <=8mm, bend radii = 4D, Cable diameters >8 <=12 mm bend radii 5D, Cable diameter >12mm bend radii 6D.

Definition of D = the overal diameter of round cable or the smaller dimension of flat cables.

To me this last bit is the important point and it basis its requirement on the minor axis.
Using osg 2.5mm twin earth (9.7 x 5.5) gives minimum bend radii = 29.1mm.
Using BS7540-1 gives minimum bend radii = 21.6 mm.
So we have a difference!! any one like to comment about which takes priority.
Or have I got it ompletely wrong.
 29 March 2007 10:40 AM
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khales

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press, thanks for that most useful reply. I think I am getting somewhere near a conclusion now, and it has to do with ambiguity in the OSG, etc. and the way I have been reading these sources.
Referring to this manufacturer's data sheet: http://www.batt.co.uk/images/pics/73516242y.pdf, it seems clear that one first uses the larger dimension to determine whether or not a 3x or 4x the overall diameter factor is to be used, then applies this to either the major or minor of the two stated overall diameters (as appropriate) to determine the minimum alowable radius. That makes sense to me, and I'm going to settle for that. Problem solved!
Looking back now at the OSG, etc., I see that it was always possible (but it hadn't occured to me) to read them in the same way. (Now I feel silly; still nobody here spotted my simple misunderstanding!)

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khales
 29 March 2007 12:04 PM
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davezawadi

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Does this mean you are still going to hand out code 2s for mini trunking, or even surface wiring? If so I think you are heading for trouble as my reading of the data sheet is not quite the same as yours. I see the minimum bend radius as a guarantee of compliance with 522-08-03 and nothing else, in the same way as the maximum operating temperature.

In my view codes can only result from non-compliance with a direct regulation.

Regards
David

-------------------------
David
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 29 March 2007 12:58 PM
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khales

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Originally posted by: davezawadi
Does this mean you are still going to hand out code 2s ...

Oh no! I've become much more tolerant as a result of this discussion. From now on, if I spot a cable bent below the minimum radius recommended in the OSG or ESC Technical Handbook, then I shall invite the installer to provide proof that no cable damage has occured and/or evidence that it does not contravene the manufacturer's recommendations - only in the (unlikely) event that this is not forthcoming will I contemplate a code-2.

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khales
 29 March 2007 01:49 PM
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davezawadi

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No, that doesn't work, quite reasonably they would charge you for the privilege! You are the inspector and if you can't see any damage, and the EIC says there is no electrical problem, thats it. It doesn't matter what other document you may wish to reference, the call is yours!

This idea that someone (the DUTY holder) may attempt to put his responsibility on someone elses shoulders is COMPLETELY wrong. I know government ministers try to do it, but you are in my view not a competent person if you take such a course! If you cannot inspect the installation properly yourself you should not do it, and should hold no view on it at all.

You made no mention of T&E inside other mechanical protection, and this is the question you wanted answered. Was my point insufficiently clear?

Regards
David

-------------------------
David
CEng etc, don't ask, its a result not a question!
 30 March 2007 10:36 AM
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khales

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I appologise for my feeble attempt at sarcastic wit, which was not appreciated and was probaby inappropriate for this forum.

You are quite right that an inspector has responsibilities that must be exercised, and not avoided. Let me be clear about how this works here. I understand that things may be done differently where you are.

First of all, the primary duty of our inspector is to inspect with reference to the design - which may include departures from BS7671 specified by the Designer.
During an inspection to BS7671, we seek to identify any cables that have been bent in contravention of 522-08-03; which raises the question of what constitutes cable damage in this context. Since damage to sheath, insulation or conductors may be internal, and therefore not evident to visual inspection, we refer to recommended minimum bending radii in accordance with British Standards or the cable manufacturer's specification or, where there is no such information, to values that represent accepted practice. In the field, we use the table in the OSG, or the table in the ESC Techncial Handbook in the first instance as representative of these sources.
Now, here is a key point: if the cable has been bent below the recommended minimum radius then we consider that is, of itself, an instance of damage to the cable. To put it another way: we consider that a cable that has a sharp bend (especially beyond manufacturer's specifications) IS a damaged cable. It WILL receive a code-2.

It is hypothetically possible that the 'designer' may require or confirm (e.g. in retrospect) that cables are to be bent below the recommended minimum radii. We would require that to be notified to us in writing. Where the designer believes this will result in a departure from BS7671, then that should be stated on the certicate by the designer - otherwise we would state it as a limitation imposed on our inspection. In these circumstances I would expect our procedure to be to look out for any visible signs of damage to the sheath - including wrinkling, stretching, splits, signs of excessive strain (such as colour change in sheath material), that give us concern and report these. A code-2 (evident damage) or code-3 (requires reference to the cable manufacturer or other authority) would be appropriate.

Now, as to cables in trunking, including mini-trunking. Bends in cables (sheathed or un-sheathed) in trunking are inspected using the same criteria. It is not our responsibility to decide that actual or potential cable damage does not matter because it is contained within trunking. Designers and Installers have several options for avoiding problems at inspection: including the use of un-sheathed cables (singles) - which have smaller recommended bending radii, or removal of the sheath at bends.

Installers with whom we work know that we will look critically at cable bends, and how to avoid problems by taking appropriate care during installation.

I do not claim that this is the only possible approach to the topic; but it works (so far) for us. Others may prefer different arrangements.

-------------------------
khales
 30 March 2007 11:11 AM
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GeoffBlackwell

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Khales
Let me be clear about how this works here. I understand that things may be done differently where you are.

Just out of interest, where is here?

Khales
In the field, we use the table in the OSG, or the table in the ESC Techncial Handbook in the first instance as representative of these sources.

Be careful when using these sources - particularly if it is in a situation where you may be challenged. The use of a BS or manufacturer's instructions is far safer. You have no guarantee that these other bodies are right or that they will back up their assertions. They are only expressing their opinions.

I am also interested to know why there is such an apparent emphasis on damage to cables through excessive bending - have you had experience of cable failure because of this?

Finally if you choose to use code 2 then you are requiring that this defect be corrected and since this may involve rewiring significant sections of an installation, at possibly considerable expense, you must be ready to stand your ground. It is not inconceivable that you could end up in front of a court - this is not a good place to resolve technical issues. However, if you are sure of your ground then you must stand firm.

Regards

Geoff Blackwell
 30 March 2007 01:48 PM
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davezawadi

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Its amazing how things turn out, that nothing means what it says any more. Inspection no longer means what can be observed using senses or instruments (the BS7671 definition) but what may be felt by an unknown person, who may have it published but takes no responsibility in any case (OSG or manufacturers data E&OE). So the inspector is now not the arbiter of his report, which is made against a set of specific regulations; but subject to vague psuedo documentation which is largely meaningless!

Using Khales criteria it is not possible to inspect any installation. To do so would require ALL of the data which every manufacturer had published, the exact date on which every item was manufactured to match up the paperwork and an unbelievable quantity of time. You would also need to know that cables had not been overbent during installation and straightened afterwards! And of course a million other things.

This kind of stuff used to be insisted on for military and some other government projects. It was found to actually reduce quality, simply because the number of places where something could go wrong became vast! It used to be ruinously expensive, but as the taxpayer met the bill that aspect was ignored.

I'm actually becoming quite worried by this thread, because I may end up paying for the results of this kind of "inspection", both the cost and the way customers who have experienced this may stay away from inspections at all. Certainly if you tried this kind of thing on me there would be a serious contract dispute, and as its not to BS7671 you would pay very dearly! Just to be clear, to win on this point you would have to show (the balance of probabilities critieria) that BS7671 was inadequate in this respect AND that bending cables as you have described damages them in a was which is not sensed by a competent inspector. I'm sure you could not do so. Just saying you were unsure is entirely inadequate, it demonstrates a lack of competence which would be fatal to your case.

I think that some of this attitude may be coming from a few high profile cases in the rail industry where accidents are blamed on poor inspection. I have no idea if this is really the cause, but I feel that the inspectors may be being blamed for things beyond their control. This blame system is part of the "spin" which has become endemic everywhere, it is because authority and responsibility have been separated for political ends. They are one and the same in any functioning society.

If you issue a BS7671 EIC, PIR etc surely it must refer to those regulations and methods. If you want to do something different I suggest you make up your own rules and forms. Of course no one in their right mind would take any notice of what you said, because you would have denied the authority and responsibility of the BSI, the IEE etc to produce satisfactory regulations.

I'm puzzled as to why this radius is so important to you, perhaps you have some proof that what you are doing is worthwhile, what accident does it prevent? What proof have you, perhaps we should put it in the 17th edition?

David

-------------------------
David
CEng etc, don't ask, its a result not a question!
IET » Wiring and the regulations » Minimum bending radius in T&E cable

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