Pig tailing Paired screened cable

[WaterSparky]

Hi guys,

Was talking with a friend of mine, thats been snagged on on pig-tailing pair screened and overall screened cable before the actual cable enters the junction box (In this case a Rotork, with 2 x 4-20mA loops) His Commissioning engineer says its not allowed, I come from a maintenance background and find this brilliant as when you have to remove a instrument or actuator etc the pig tails are a god send. Essentially Im asking the question of what effect this would have, I can't see it being too bad seeing as it is Paired, Pair screened and overall screened? Ive done this nearly every time and never had a problem.

Regards

WS

 

[shanky887614]

his commissioning engineer is a bellend.

nothing wrong with it, we had to do it by the book when we did the training room and most controls were pigtailed.

we are talking we needed castle glands,metal tywraps, most containment types and most different gland types.

the guy was ott really, 1mm out of the end of all ferules etc

 

[Knobhead]

It depends on what the specification is. The commissioning team have to stick to it and will make sure you do.

 

[Bobster]

I've never seen an issue with it.

I have seen huge problems caused with people pigtailing profibus cable.

 

[bignod]

In work with me we never pigtail cables into rotork's or any other type of actuator. It is on a power station and have nearly 1000 actuators on site. Find it easier for fault finding if the cables are not pigtailed

 

[Bobster]

How does a pigtail hinder fault finding?

 

[shanky887614]

surely you are checking via the cores for electrical problems.

one of the reasons we use pig tails is, if there is a problem on the damper itself the mechanical guys/estates can remove it without turning off the panel or disconnecting the cables out of the panel

 

[bignod]

It is due partially due to our control schemes and due to cable runs picking up induced voltages even with screened cables

 

[shanky887614]

It is due partially due to our control schemes and due to cable runs picking up induced voltages even with screened cables

i dont buy that, if a small pig tail going into an actuator can pick up induced voltage you have bigger problems.


the screened cable should be kept seperate from power anyway so what are you guys doing?

 

[bignod]

A lot of cables share the same cable ways from 11kv to control cables on trays in same cable ways. It is the way the station was designed.can be a real headache on times following cables and tracing faults. Not pigtailing was a standard when the place was built 40 years ago.

 

[shanky887614]

A lot of cables share the same cable ways from 11kv to control cables on trays in same cable ways. It is the way the station was designed.can be a real headache on times following cables and tracing faults. Not pigtailing was a standard when the place was built 40 years ago.

nothing to say you shouldn't in regs then, im sure in regs its states segragation of circuits.


or are your controls 230v? we tend to use 12/24v controls.



fair enough if thats how it was done, not much you can do

 

[bignod]

Our controls can be 240 ac or DC down to 24v but are predominantly 50v DC . Being a power station we don't have to comply with the regulations, but use them as a best practice

 

[Bobster]

Dependent on the type of power station you work at, you will have to work to far more strict regulations. They aren't just best practice!

 

[Knobhead]

surely you are checking via the cores for electrical problems.

one of the reasons we use pig tails is, if there is a problem on the damper itself the mechanical guys/estates can remove it without turning off the panel or disconnecting the cables out of the panel

I wouldn’t buy that either. If an actuator was to be changed the valve gets changed. I don’t know what size Rotork’s the OP is on about but with most of ours the cables would get damaged unless disconnected.

It still comes down to what is in the specification and not what you think it should be.

 

[shanky887614]

I wouldn’t buy that either. If an actuator was to be changed the valve gets changed. I don’t know what size Rotork’s the OP is on about but with most of ours the cables would get damaged unless disconnected.

It still comes down to what is in the specification and not what you think it should be.

in our case quite often pipe fitters will change valves etc and that is one of the reasons we pigtail them.

 

[Jamzie]

It is due partially due to our control schemes and due to cable runs picking up induced voltages even with screened cables

Are the screened cables earthed at both ends?

 

[bomjac]

Could be an issue with support of the cable (or rather lack of) where it enters the rotork. I have known some companies specify that there must be a cable support no more than 6" from any gland. If you are on water works you potentially have issues with condensation or even plain old water entering through a deformed outer seal on the gland.

The weight of the pigtail can deform the outer seal to cause this if it is not supported.

It is up to commissioning to show you exactly why it is not allowed or else your friend should put in a query to engineerng asking them to clarify. If your friend has installed.to spec the it is up to the client to instruct you to remove them all and gland direct. If it is not to spec then the installers have a major headache if it comes to re gland in and testing everything.

Very often "preferential engineering" wil be sought by the inspection team rather than the agreed contractual minimum.

 

[shanky887614]

Could be an issue with support of the cable (or rather lack of) where it enters the rotork. I have known some companies specify that there must be a cable support no more than 6" from any gland. If you are on water works you potentially have issues with condensation or even plain old water entering through a deformed outer seal on the gland.

The weight of the pigtail can deform the outer seal to cause this if it is not supported.

It is up to commissioning to show you exactly why it is not allowed or else your friend should put in a query to engineerng asking them to clarify. If your friend has installed.to spec the it is up to the client to instruct you to remove them all and gland direct. If it is not to spec then the installers have a major headache if it comes to re gland in and testing everything.

Very often "preferential engineering" wil be sought by the inspection team rather than the agreed contractual minimum.

sorry but screened cable like this for sensors etc will be say 0.75/1mm. a small pigtail wont deform the right sized gland/stuffing gland.

 

[WaterSparky]

Bit late guys, but thank you for the replies none the less! I`m not sure what size rotork it was ( Ill talk to him) I find the pigtails are useful when Mechanics are changing out faulty valve etc so it frees up an ICA/Elec. For me its not so useful but as some are in awful places and allows me to wire it up prior to placing on the valve. Each to their own, we have our on standards in-line or succeeding the WIMES standards. But I'm unsure if this will come into it at all.

Edit: In reply/support like Shanky says these will only be 0.75mm2 2 pair or 2x0.75mm2 1 pair so doubtful it will deform, and the supply to the rotork

Thanks guys
WS