Just interested really.

[shaky1105]

Hi guys, just wondered if anyone can explain this, or indeed if there's anything to explain !

We are presently wiring telemetry lines up for BT at their new Roadside DSLAM cabinets, basically consisting of a 5pr comms cable between the new dslam cabinet and the existing PCP cabinet.

Part of the job is that if the cabinets are closer than 3M to each other, we have to also run a 10mm bonding cable between them ( The dslam is mains powered ) for electrical safety.

The thing that strikes me as strange is that this cable terminates directly onto the shell of the two cabinets but then we also run a 6mm bonding cable to the two doors of the existing Street PCP Cabinet and I just wondered what the reduction in csa of the door bonds would be for - if anyone knows ?

Just being nosy really

Cheers

 

[JohnW2]

Part of the job is that if the cabinets are closer than 3M to each other, we have to also run a 10mm bonding cable between them ( The dslam is mains powered ) for electrical safety.
The thing that strikes me as strange is that this cable terminates directly onto the shell of the two cabinets but then we also run a 6mm bonding cable to the two doors of the existing Street PCP Cabinet and I just wondered what the reduction in csa of the door bonds would be for - if anyone knows ?

Who knows - maybe just 'history', perhaps. Also, I presume that the 6mm² to the doors is only 'supplementary' to probable continuity through metal hinges - which might be a reason for smaller CSA being regarded as acceptable ... and then there's the practical consideration than 10mm² cable might be more of a hindrance to opening/closing of the doors (and hence more likely to break?!).

Someone may well have a more informed answer.

Kind REgards,
John

 

[shaky1105]

Who knows - maybe just 'history', perhaps. Also, I presume that the 6mm² to the doors is only 'supplementary' to probable continuity through metal hinges - which might be a reason for smaller CSA being regarded as acceptable ... and then there's the practical consideration than 10mm² cable might be more of a hindrance to opening/closing of the doors (and hence more likely to break?!).

Well, that does make sense I reckon. The Earth Cable between the cabinets is 6491x stuff but the Door Bonds have much more strands in them, making them a lot more flexible.

I forgot to add that the dslam cabinet is fed via a 25mm split conc cable, then via 25mm tails to the meter and finally what looks like Tri-Rated 25mm into the Power termination of the cabinet, if that makes any difference. The dslam cabinet also has it's own Earth Rod

 

[JohnW2]

Well, that does make sense I reckon. The Earth Cable between the cabinets is 6491x stuff but the Door Bonds have much more strands in them, making them a lot more flexible.
I forgot to add that the dslam cabinet is fed via a 25mm split conc cable, then via 25mm tails to the meter and finally what looks like Tri-Rated 25mm into the Power termination of the cabinet, if that makes any difference. The dslam cabinet also has it's own Earth Rod

I suppose another (and perhaps better) way of looking at this would be by analogy with a, say, residential electrical installation. The cable between the cabinets could be regarded as analogous with 'main protective bonding', and could under some circumstances end up carrying fault currents. On the other hand, the bonding to the doors is more analogous to 'supplementary bonding', serving primarily to reduce touch voltages, and very unlikely to carry a fault current. That might be a more credible explanation for the differing CSA requirements.

Kind Regards, John.

 

[sparkticus]

I suppose another (and perhaps better) way of looking at this would be by analogy with a, say, residential electrical installation. The cable between the cabinets could be regarded as analogous with 'main protective bonding', and could under some circumstances end up carrying fault currents. On the other hand, the bonding to the doors is more analogous to 'supplementary bonding', serving primarily to reduce touch voltages, and very unlikely to carry a fault current. That might be a more credible explanation for the differing CSA requirements.

I believe it is almost certainly for reasons that the door is unlikely to be a path for direct fault current. I quite regularly see 6mm G/Y on doors of metal utility, fuse, isolator, contactor and distribution enclosures.

 

[JohnW2]

I believe it is almost certainly for reasons that the door is unlikely to be a path for direct fault current. I quite regularly see 6mm G/Y on doors of metal utility, fuse, isolator, contactor and distribution enclosures.

Indeed - as you will have seen, that's the conclusion I more-or-less came to in the 'second breath' of my thinking! An irony, of course, is that (depending on the design of the hinges) the bonding to the doors might end up with a larger effective CSA than the 'main' bonding between the cabinets.

Mind you, 6mm² as bonding just to offer 'touch voltage protection' seems to be 'erring on the side of caution' - given the CSAs required for supplementary bonding (when required) in domestical installations.

Kind Regards, John.

 

[sparkticus]

Indeed - as you will have seen, that's the conclusion I more-or-less came to in the 'second breath' of my thinking! An irony, of course, is that (depending on the design of the hinges) the bonding to the doors might end up with a larger effective CSA than the 'main' bonding between the cabinets.

Yes, depending on the hinge design - some of them welded with unpainted robust interleaved piano style - it would be hard to improve that with even a 16mm wire link.

What is interesting is some of the metal box enclosed electronics (as opposed to electrical) systems have "supplementary bonding" on the doors with the primary intention of noise/esd reduction. Which can sometimes be quite naive because having tried to "screen out" a distant transmitter noise (signal from a distant TV xmitter in Tromso Norway) from penetrating a metal enclosed semiconductor test rig while looking for "shot noise" using a 60db amplifier, I can clearly state that it is more about preventing "ground loops" than screening. You would not believe how difficult it can be to screen out unwanted weak RF signals!

 

[JohnW2]

Yes, depending on the hinge design - some of them welded with unpainted robust interleaved piano style - it would be hard to improve that with even a 16mm wire link.

Indeed.

.... Which can sometimes be quite naive because having tried to "screen out" a distant transmitter noise (signal from a distant TV xmitter in Tromso Norway) from penetrating a metal enclosed semiconductor test rig while looking for "shot noise" using a 60db amplifier, I can clearly state that it is more about preventing "ground loops" than screening. You would not believe how difficult it can be to screen out unwanted weak RF signals!

You take me back a few decades but, yes, RF screening can be a nightmare. In the context of this discussion, the earthing ('supplementary bonding') of the screening is often not of much consequence (the metal itself doing the screening). However, I seem to recall that the biggest game of all was that, if the electronics being screened were, themselves, RF circuitry, attachment of the screening often de-tuned one's circuitry (and prevented access to re-tune it!) - leading to extremely tedious 'progressive re-tuning' exercises. Life is a bit easier when one is trying to keep RF out of DC or low frequency circuitry, but I nevertheless remember lots of heartache trying to keep Radio 2 (or was it 'The Light Programme') out of circuitry processing microvolt bio-electrical signals!

Kind Regards, John.

 

[Astra99]

The Light Programme. 1500m Long Wave (or 200kHz in new-speak). Takes me back a day or several!

 

[sparkticus]

The Light Programme. 1500m Long Wave (or 200kHz in new-speak). Takes me back a day or several!

ahhh, how I miss those days of wavelength rather than the contemporary "frequency"

 

[JohnW2]

The Light Programme. 1500m Long Wave (or 200kHz in new-speak). Takes me back a day or several!

That's the one - and it ran horrendous power, feeding acres or aerials/antennae. ...and, because of it's very wide coverage, carried the standard time signals, IIRC (possibly still does!) ... and (again!), because it was precisely 200kHz, was widely used for checking/trimming frequency measuring equipment..

Kind Regards, John.

 

[JohnW2]

ahhh, how I miss those days of wavelength rather than the contemporary "frequency"

Quite so - nostalgia is quite comforting at times

I'm slightly intrigued by the fact that, once one gets below a wavelength of about 10cm (frequency 3GHz), 'frequency' is no longer contemporary - hence IR, visible light, UV and X-rays etc are still usually characterised by wavelength, rather than frequency. I wonder why?

Kind Regards, John.

 

[sparkticus]

Life is a bit easier when one is trying to keep RF out of DC or low frequency circuitry, but I nevertheless remember lots of heartache trying to keep Radio 2 (or was it 'The Light Programme') out of circuitry processing microvolt bio-electrical signals!

Yes, looking for shot noise (in doped silicon) you are usually/often down in the femto-amp range (10 to the minus 15) using a sweep generator set at very low voltage (often in the micro or 10s of nano volt range) You would think therefore that "ignoring" a huge RF induced current would be easy but it is the external influence raising the noise floor that becomes the aggravating factor. Probably the same aggravation while trying to sense (and amplify) tiny biological signals from "nudging molecules" or even intra/extracellular potentials formed during nerve depolarisation though I expect (but am not sure) that peripheral nerve depolarisation potentials (and resultant signals) are quite large compared to say signals in nerve axons of the brain or inter-molecular signals?

 

[sparkticus]

ahhh, how I miss those days of wavelength rather than the contemporary "frequency"

Quite so - nostalgia is quite comforting at times

I'm slightly intrigued by the fact that, once one gets below a wavelength of about 10cm (frequency 3GHz), 'frequency' is no longer contemporary - hence IR, visible light, UV and X-rays etc are still usually characterised by wavelength, rather than frequency. I wonder why?

That's a good question. I used to be very involved with characterising emission of light from semiconductor devices and we always talked in terms of 400 nano-meters - 600 nano-meters etc and not frequency which is ironic because we had to convert to energy which involves "hF" (planck) so we had to take a slightly longer rout with h*c/ wavelength. We also had to integrate multiple emissions at various wavelengths to find the total energy (energy under the curve as it were) and for that we switched to frequency I suspect though it is just historic. A bit like ordering 3 meters of 4X2 timber

 

[JohnW2]

That's a good question. ... characterising emission of light from semiconductor devices and we always talked in terms of 400 nano-meters - 600 nano-meters etc and not frequency .... I suspect though it is just historic.

Maybe, although it's far from obvious why 'historic' stuck with wavelengths below 10cm but not those above ... and, of course, you've missed out a step of history - with the longer wavelengths, we first went through the phase of specifying their frequency in cycles/sec (which seemed so logical) before we ended up with Hz!

A bit like ordering 3 meters of 4X2 timber

I still do, if I have a chance - and our local village butcher doesn't understand or recognise the metric system (well, his scales do, but he then converts into 'old money' to work out the price!)

Kind Regards, John.