SY, CY None Compliant????

[Wilko]

I was asked this many years ago and never got a definitive answer. My opinion is the braid should be earthed but can you give any info as to where this is stated, is it staring me in the face somewhere?

Hi - FWIW I think it should be earthed, too. All I have so far is 131.2.1 "persons and livestock shall be protected against dangers .from. contact with live parts ..." and by earthing the braid it should reduce this risk, I think.

 

[westward10]

Hi - FWIW I think it should be earthed, too. All I have so far is 131.2.1 "persons and livestock shall be protected against dangers .from. contact with live parts ..." and by earthing the braid it should reduce this risk, I think.

The problem I always found it is not recognised by BS7671.

 

[Wilko]

Yes, Ruston's post #31 had a link to Eland Cables and I quote them :
"SY control cables are not suitable for fixed wiring applications requiring compliance with the requirements set out in BS7671"

 

[westward10]

Yes, Ruston's post #31 had a link to Eland Cables and I quote them :
"SY control cables are not suitable for fixed wiring applications requiring compliance with the requirements set out in BS7671"

Yeah just looked at that. Must be over 10 years ago I looked into it and gave up, told client I had exhausted all avenues.

 

[Richard Burns]

I would take the approach that the cabling system is live parts in close proximity to a piece of metal that in a fault is almost certain to contact live parts and become live and by the nature of the cable could also be touched at points and would therefore be an exposed conductive part and require earthing.
This avoids all problems about the cable being recognised by BS7671 and just considers the situation in place.

 

[westward10]

I would take the approach that the cabling system is live parts in close proximity to a piece of metal that in a fault is almost certain to contact live parts and become live and by the nature of the cable could also be touched at points and would therefore be an exposed conductive part and require earthing.
This avoids all problems about the cable being recognised by BS7671 and just considers the situation in place.

I totally agree but I have been asked where it says this in black and white in the past. Is it really an exposed conductive part? This is another issue which surrounded the earthing problem.

 

[Richard Burns]

I totally agree but I have been asked where it says this in black and white in the past. Is it really an exposed conductive part? This is another issue which surrounded the earthing problem.

Then you tell them, I have given my professional opinion if you do not like it then choose someone else!

 

[sunray]

Pigtails are not suitable for EMC compliance, you need to use the correct gland or terminate it into the VSD chassis clamps.

Also the screen should be connected from the VSD chassis back to the motor frame.

Edit: and another thing. Seeing you cannot buy a concentric SY (3 core without a green yellow core) it's not really a suitable VSD cable at all.

Having worked on a VSD today the instructions clearly stated the screen is not to be terminated at the motor but an earth conductor must be provided... What the difference is I have not a scooby doo.

There are 2 earth points between the motor connexions and the VSD cable clamp, and listed for the earth conductor and screen.

Cable being 4 core 6mm² SY.

The reason I was there being a brand new motor (identical make and spec to original) created interference on the BMS in the form of silly 0-10V and 4-20mA readings on nearby sensors... Not an issue with the other motor in the pair or apparently the removed motor.

The only work done being the replacement of 18KW motor by the company maintenance team.

My first check being the 2 worst affected sensor wiring as the maintenance team informed me they had thouroughly checked the motor wiring, both cabled with 0.5mm² SY, plastic cable glanded into BMS enclosure, screen very short pigtailed to earth bar, one sensor in 5 core for 0V, 24Vdc, earth & 0-10V loop. Second being 2 core for 4-20mA. At sensor ends stuffing glanded to sensor, the screens being well isolated. All nicely done and untouched for years.

My next check being the motor wiring, I looked in the VSD first as the cover had been removed; green yellow terminated in one terminal screen pigtailed into other terminal, metal clamp on plastic sheath (only 10mm from earth terminal).
At motor the very brown SY was terminated with new looking brass gland into motor terminal box, the other motor of the pair sported a grey gland which I checked with a meter to be insulating (presumably plastic).
Internally all 4 wires were correctly terminated with new yellow ring crimps onto their studs.

Luckily the brass gland was fitted with locknut and undoing it there was plenty of slack to slide the gland away from the metal enclosure and protecting the wires from chaffing with card the motor was started and ran without altering the sensor readings.

Luckily I had a suitable 25mm stuffing gland and replacing the new brass gland was a fairly easy, maintenance had heatshrink and heat gun to add extra protection to the screen after trimming it back to inside the sheath.

I'd forgotten this thread and found it while looking to see if the subject had been raised here.

 

[LeeH]

I’ll find the reference today when I get to not the office, it was 7 years ago though and I don’t have the literature anymore due to a job change. I did extensive research on an interesting job that had some ATEX zones and had a crap load of drives, finding a cable to suit both EMC compliance and ATEX compliance was impossible.

But basically the ideal cable has an external CPC as the twists have a substantial effect. The 4th core inhibits this.

Using the EMC clamp on the insulation and pig tailing the braid is a bit dumb though.

Anyway, I used SWA in the end and rode off into the sunset on my horse as the perfect cable does not exists and I couldn’t bring myself to use flexible cables in that environment.

 

[LeeH]

Did the drive have an EMC filter? Only the high end ones have the integral. (Or did 10 years ago)

 

[LeeH]

@sunray An edited snippet below regarding grounding at one end and the use of pigtails.

This requirement to employ ‘good EMC
engineering practices having regard to
the state of the art’, is a big problem for
electrical engineers, etc., since most
of them seem to believe that all that is
required for good EMC engineering is to
use single-point earthing (grounding)
terminate cable shields at one end only,

These might possibly have been
acceptable EMC practices in the 1950s,
when FM Radio and Television at VHF
were new and considered to be the
pinnacle of high technology, and digital
circuits and software were not even on
the horizon. But they are generally bad
EMC engineering practices these days

Remember that pigtailed shields are
never going to be much use for EMC

https://www.emcstandards.co.uk/files/reo_guide_to_fixed_installation_best_practice.pdf

Also cable suggestions form the ATV930 drive manuals that we used. So its the core symmetry, not the twists. Bit instrumental cables have twisted pairs for this reason.

 

[sunray]

I think another reply is warranted here.
I've just got home from a callout to a plant room where a pair of fan motors and their control gear have been changed. Including VSD's, Estop, contactors, air louvres etc.
Job started 1800 on Friday with the expectation it would take 12 hours for a crew of 4.
One of the companies I've subbed to got the call 1100 this morning that there was problems with the BMS system with corrupted readings.
The boss called me as the 2nd hand.
We arrived around 1300 to find what I would describe as a mess, the motors are 37KW and starting either was sending sensor readings haywire. The crux of the matter is: Despite the VSD instructions clearly stating in bold text to use SWA or SY and not connecting the screen at the motor the existing 7 core 4mm Sy cables (used as doubled conductors) had been terminated using SWA glands into the motors terminal box.
Isolating the screen at the motor immediately stopped the interference although the SY earth wire is connected both ends.
The 2 of us were there for 6 hours sorting out the mess the botchers had created.

 

[Darkwood]

I think another reply is warranted here.
I've just got home from a callout to a plant room where a pair of fan motors and their control gear have been changed. Including VSD's, Estop, contactors, air louvres etc.
Job started 1800 on Friday with the expectation it would take 12 hours for a crew of 4.
One of the companies I've subbed to got the call 1100 this morning that there was problems with the BMS system with corrupted readings.
The boss called me as the 2nd hand.
We arrived around 1300 to find what I would describe as a mess, the motors are 37KW and starting either was sending sensor readings haywire. The crux of the matter is: Despite the VSD instructions clearly stating in bold text to use SWA or SY and not connecting the screen at the motor the existing 7 core 4mm Sy cables (used as doubled conductors) had been terminated using SWA glands into the motors terminal box.
Isolating the screen at the motor immediately stopped the interference although the SY earth wire is connected both ends.
The 2 of us were there for 6 hours sorting out the mess the botchers had created.

That's strange from a vsd, usually the screen is grounded at vsd and motor end, two queries, why 7 core and what vsd did you use given the screen is usually there to block high frequency radiation interfering with external electronics.

 

[sunray]

That's strange from a vsd, usually the screen is grounded at vsd and motor end, two queries, why 7 core and what vsd did you use given the screen is usually there to block high frequency radiation interfering with external electronics.

The manual clearly showed the screen isolated at the motor end, just as I have seen with other VSDs instructions.
I suspect the original motor was running star delta, requiring 6+earth, the removed VSD's were Square D.
I truly didn't note the make of the new drives, the grey plastic covers were off and laying on the floor and proped against wall so I never saw the fronts. The guys said they are American if it gives any clues. and they only had a few well worn pages of the manual printed off.

This is not the first time I've encountered the same interference problem when the screen is connected at the motor end. Diagnosing the problem was quite quick once we got to it, realistically it took a lot longer than it should have done as the guy in charge was insistant they had done their work to MIs and the initial conversation was very protracted as he tried to direct us towards the sensors and The "ancient Trend 5h1t".

Judging by the mucky state of the guys on site, and their warnings, we were not in too much of a rush to head into the mucky plant room and looked at the VSDs T& rend IQ3 modules and wiring first, including the new 0.5mm SY control cables they had installed.

 

[Darkwood]

In different drive/motor systems you can find you get circulating currents generated and can cause other issues notedly shaft pitting from the bearings as the currents arcs from the rotating ball bearings, I have witnessed this myself on a drive setup that did require the screen to be grounded to the motor frame, however when we discussed with the drive manufacturers their response was of surprise and suggested fitting insulated bearings, the modern vsd I use has no such issues even though its grounded at both ends.
Tbh the difference between drives requiring grounding both ends and those requiring isolation at the motor is beyond me so I just do as instructed by the drive manufacturers guidance.