Torque Talk

[Marvo]

Long time no see and I hope you're all doing well.

I see that torque screwdrivers have become the height of fashion in the UK over the last couple of years. I wondered what general practice has been adopted regarding their use.

Are electricians expected to;
Have a torque screwdriver on site at all times?
Check torque of all terminations in CU's / DB's they open or just any new terminations they make?
Check the torque of all terminations in any accessories they open?

Also;
Is acceptable to do torque testing live in a CU (assuming a VDE screwdriver)? Is a risk assessment required for live torque testing?
If torque testing is performed in a DB/CU do you just check screws for tightness or do you loosen off all terminations first then re-torque to the correct figure to eliminate overtightened connections as well as loose ones?

 

[Dustydazzler]

I used to have a torque wrench , I be danged if I know what happened to it thou

One a bit like this

https://www.amazon.co.uk/Draper-30357-Square-Drive-22-1-154-9lb-ft/dp/B0001K9S52/ref=asc_df_B0001K9S52?tag=bingshoppinga-21&linkCode=df0&hvadid=79920808416455&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583520383274666&psc=1

 

[pc1966]

I think most of the key points have been said.

Terminals should be correctly tightened but the regs don't specify how that is done, however, nobody can argue that using the manufacturer's torque setting is not a good way of doing so and thus meeting the fundamental requirement of a sound connection. To avoid liability then proving you have the tool to do so than comes along.

As an aside, wheel nuts and terminals both have forces making them come apart, in the case of electrical stuff though it is usually thermal cycling and not heavy vibration. What is very different is the general condition of them with terminals generally clean & dry and wheel nut varying a lot in terms of surface condition (rust through to well-greased).

Tightening by torque is not that precise as while the tool may be accurate to a few percent, the thread and fastener head friction can vary a lot depending on its finish (machining / plating quality, etc) and state of lubrication. From memory to around 30% or so. That is why high stress bolts like car cylinder heads, etc, now have other methods of final tightening such as going to a set torque to bed them down and then a given final turn angle so the bolt/stud has a certain degree of elongation and thus preload.

Another approach that is good is to observe the torque versus angle do determine how much yield there is, oddly enough that is basically the traditional craftsman's method found out by years of experience (and often a couple of failures along the way)! More summary here:

Methods of Tightening Threaded Fasteners

Personally I use a torque tool for anything critical, but for small DIN screws, etc, usually just hand feel of experience. I have a VDE screwdriver for doing MCBs, etc, and also a few non-insulated tools left over from DIY on car's etc over my lifetime that get used for the occasional big lug bolts, etc.

In my own very limited experience, Hager, Wylex, and Schneider stuff survived fine at their stated torque, one CPN device started to come apart at the seams so lesson learned - don't use them or take it slightly easier!

 

[nicebutdim]

Spot on.

Also wheel nuts are subject to significant forces and vibration making torque settings important. Whereas Domestic Electrical installations are (generally) not subject to these forces & vibrations, and so terminal screws, if nipped up, are highly unlikely to work loose.

Screw terminals are subject to thermal creep, through which they can become loose, although that's not going to be much of an issue at the time of installation.

 

[slartybartfast]

Screw terminals are subject to thermal creep, through which they can become loose, although that's not going to be much of an issue at the time of installation.

hmmm....if temperatures are getting high enough to that then cable is likely undersized

 

[DPG]

hmmm....if temperatures are getting high enough to that then cable is likely undersized

Happens at moderate temperatures, especially when stranded cables have been tinned with solder. That is why they shouldn't be tinned.

Doesn't necessarily need such high temperatures for it occur.

 

[slartybartfast]

Happens at moderate temperatures, especially when stranded cables have been tinned with solder. That is why they shouldn't be tinned.

Doesn't necessarily need such high temperatures for it occur.

Ferrules on stranded cables will be mandated in regs before long for sure !

 

[DPG]

Ferrules on stranded cables will be mandated in regs before long for sure !

I think they are mentioned in the regs for fine stranded wires aren't they?

 

[pc1966]

I think they are mentioned in the regs for fine stranded wires aren't they?

Not exactly, but 526.9.1 says the ends have to be "suitably treated" and ferrules are one way to do that. The other common way is the ultrasonic weld you see on some RCBO neutral tails that are fine-stranded.

Also that section (526.9.2/3) say no to tinning conductors for screw terminals, etc.

Soldering is great though if it is actually the means of connection!

 

[pc1966]

hmmm....if temperatures are getting high enough to that then cable is likely undersized

It is more the cycling of hot/cold you get with time-varying loads or, to a lesser degree, just from day/night temperature changes, etc.

 

[Rockingit]

Not exactly, but 526.9.1 says the ends have to be "suitably treated" and ferrules are one way to do that. The other common way is the ultrasonic weld you see on some RCBO neutral tails that are fine-stranded.

Also that section (526.9.2/3) say no to tinning conductors for screw terminals, etc.

Soldering is great though if it is actually the means of connection!

Indeed, but who in their right mind gets out the soldering iron station when it takes 2s to put a ferrule on?!

 

[Lucien Nunes]

In the days before ferrules were widely known outside the panel-building industry, it was quite a popular albeit misguided pastime. For example, we saw a number of small theatre lighting installations with a plug patch at the dimmers, where the flexible patch cords were tinned before connecting to the conduit cables via normal terminal block. Someone obviously tinned their way along the line of them making each and every connection a loose one.

 

[pc1966]

As above, was quite common and obviously enough for it to be added to the regs as a no-no.

Ferrules are quite a neat way of helping fine stranded wire, and occasionally I have put them on coarse stranded if it is a big screw terminal instead of cage-clamp, etc, as I have seen wires removed and looking worryingly tin and flat as a result.

 

[Other_Tucker]

I'm all for the correct tightness in theory, it winds me up when colleagues use impact drivers to tighten (permanently weld) screw connections in CU's. I'd bet the correct, recommended torque, is less than normal wrist driver tightness.

If it becomes mandatory bureaucracy, how long until you need a calibration certificate for your torque screwdriver.

 

[Rockingit]

As above, was quite common and obviously enough for it to be added to the regs as a no-no.

Ferrules are quite a neat way of helping fine stranded wire, and occasionally I have put them on coarse stranded if it is a big screw terminal instead of cage-clamp, etc, as I have seen wires removed and looking worryingly tin and flat as a result.

You only have to fix one and then take it out again to see where the screw clamped down and all the strands it missed!

 

[DPG]

I'm all for the correct tightness in theory, it winds me up when colleagues use impact drivers to tighten (permanently weld) screw connections in CU's. I'd bet the correct, recommended torque, is less than normal wrist driver tightness.

If it becomes mandatory bureaucracy, how long until you need a calibration certificate for your torque screwdriver.

You should have it calibrated regularly already.