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Topic Title: Power track
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Created On: 02 February 2013 07:49 PM
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 02 February 2013 07:49 PM
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Martynduerden

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I am looking at some tracks some of which are extremely long.

They are all on 32A mcb's the tracks are rated at 63A with the usual conditions,which are not currently met hence 32A.

What would you consider a sensible number of connections per track?

The intended use will be a 4way individually fused trailing lead per desk -supplied as part of the desk. It has a single standard 13A plug the individual fuses are 3A.

I am down for re-supplying the entire office as there is a 30% increase in desks the clients plan was 134 desks each with a 4 way on 8 tracks!

Now my concerns are I suppose obvious, surge on power up, hpcc & loading.

On average pc consumption of 250w that's 1A per user/desk.

Thoughts please

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Martyn.

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 02 February 2013 08:50 PM
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dg66

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High integrity earthing due to accumaltive earth leakage, is one thing i would consider.Or is that what hpcc means?

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Dave(not Cockburn)
 02 February 2013 09:16 PM
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Martynduerden

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Originally posted by: dg66

High integrity earthing due to accumaltive earth leakage, is one thing i would consider.Or is that what hpcc means?


Yes, thanks.

HPCC- High Protective Conductor Currents

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Martyn.

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 02 February 2013 09:39 PM
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Starfire

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Start up surge, after a power cut ?

Consider a contactor on each track with a small time delay relay or a sequencer ?

Harmonic distortion causing high neutral currents ? 1 pc per desk + the inevitable chargers etc = lots of switched mode power supplies

add another 50w per desk for the monitor(s) (maybe more than one) ?
 02 February 2013 11:54 PM
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peteTLM

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Originally posted by: Starfire

Start up surge, after a power cut ?



Consider a contactor on each track with a small time delay relay or a sequencer ?



Harmonic distortion causing high neutral currents ? 1 pc per desk + the inevitable chargers etc = lots of switched mode power supplies



add another 50w per desk for the monitor(s) (maybe more than one) ?


I think with a 32A mcb in place its going to be a problem getting the individual circuits back on to start with depending on the finer details of the power supplies on the pc's.

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 03 February 2013 12:18 AM
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Martynduerden

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Originally posted by: peteTLM

Originally posted by: Starfire

Start up surge, after a power cut ?

Consider a contactor on each track with a small time delay relay or a sequencer ?

Harmonic distortion causing high neutral currents ? 1 pc per desk + the inevitable chargers etc = lots of switched mode power supplies

add another 50w per desk for the monitor(s) (maybe more than one) ?


I think with a 32A mcb in place its going to be a problem getting the individual circuits back on to start with depending on the finer details of the power supplies on the pc's.


I don't think the 32a breaker is the issue - that part is part of a type approved system subject to 3m limits.

There are very few issues at the moment with the current setup som tracks trip on restart but not all,surprising really but theory and fact are two very different things.

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Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 03 February 2013 09:03 AM
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ebee

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"som tracks trip on restart but not all,surprising really but theory and fact are two very different things."

Slight variations accruing for the good this time.
I wonder if each startup produces the same culprits or just some of the same?

I`d definitely be thinking of sequencing delays for the sake of surges and harmonics. The fact your adding to will only make the system worse not better in this respect I should imagine.

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Ebee (M I S P N)

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 03 February 2013 11:27 AM
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Martynduerden

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Originally posted by: ebee

"som tracks trip on restart but not all,surprising really but theory and fact are two very different things."

Slight variations accruing for the good this time.

I wonder if each startup produces the same culprits or just some of the same?

I`d definitely be thinking of sequencing delays for the sake of surges and harmonics. The fact your adding to will only make the system worse not better in this respect I should imagine.


I am pushing for a full redesign, though will sequencing the individual tracks help the individual breakers ?

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Martyn.

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 03 February 2013 12:27 PM
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OMS

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I am pushing for a full redesign, though will sequencing the individual tracks help the individual breakers ?


Not sure you need a full redesign to be honest. if i was the client I'd be a bit annoyed at having initially had a flexible system installation, it's considerd unusable for the sake of adding a few more PC's.

sequencing will work, but first of all you need to decide if all the PC's wake up on ring, if they power to off after a mains failure, or are staff in the habit of leaving them on - ie is there no out of hours forced shutdown.

When you have those answers, the it's quite conceivable to have 20 + PCs on a single track - which would give you 160 desk spaces on 8 tracks.

HPCC - no problem at each desk, only the busbar is an issue - assuming metal cased with an integral earth bar, fed in armourd, then complies by default

Loading - 20 x 150W = 13A (15W/m2 - 1 person 10m2)

Loading - 20 x 250W = 20A (20w/m2 - 1 person, 12.5m2)

Loading - 20 x 300W = 24A (30w/m2 - 1 person, 10m2)

The lowest value is typical BCO guidance.

Inrush - what MCB's can you tolerate - 32A Type C or Type D would get you some reasonable inrush values for typical PC's - ie circa 15 - 20 in number. You could also deploy sequence restarters built into the power bars - ie say 50 - 755 basd on 2.5second, 5 second and 7.5 second delays scatterd across the floor plate

Going from scratch, I guess about 12 - 15 PC's (ie desk locations) wouldn't be unreasonable. It may be less if the design needs to consider resilience and potential non availibility charges

regards

OMS

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Edited: 03 February 2013 at 12:37 PM by OMS
 03 February 2013 12:51 PM
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alancapon

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Originally posted by: OMS
. . . you need to decide if all the PC's wake up on ring, if they power to off after a mains failure, or are staff in the habit of leaving them on - ie is there no out of hours forced shutdown. . .

A lot of PCs these days do not have "real" on/off switches - the power buttons actually switch off the output of the power unit, not the input. It is conceivable that the switch-on surge will be similar whether the device was left on or off, as the capacitors in the switched-mode power unit will need to charge either way.

Regards,

Alan.
 03 February 2013 01:10 PM
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OMS

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Agreed Alan, but "modern" PC's are also far less aggressive on start up - I guess I was simply suggesting that the characteristics of the load needs to be understood.

Regards

OMS

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 03 February 2013 05:17 PM
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Zs

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err, call me simplistic but I'd be suggesting some new circuits, on their own breakers, for the new desks and leaving the existing as it is right now. With reference to the new section of big green (444?) au sujet d'electromagnetic distress.

Except that you say some tracks trip on restart, but clearly not all of them....I'd want to know what is wrong with the ones that do cause a trip.

So I'd be feeding some new circuits in the floor void, which is easy, applying the new bits from big green and offering a fault-find for the trippy ones.



Zs
 03 February 2013 05:39 PM
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OMS

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Maybe Zs, but the client has a flexible underfloor busbar system - he just wants to plug in some additional desks - that's the starting point

I'd start by evaluating the existing system for capability, then move to a few additional tracks (as you suggest) if I couldn't sensibly make the existing system work. Which is probably easier than modifying the new in terms that it would be easier to "prove" and keeps your work away from the existing work

The last resort would be to rip it out and start again - and to do that the designer is going to need to robustly stand his ground against attacks from a number of directions.

8 bus bars - 134 desks - whilst I'd have liked 9 busbars ideally, it sounds to me that just plugging in the extra floor boxes or power bars isn't any kind of real problem.

Fault finding the trippy ones is probably an exercise in futility, in the sense there is no fault to find - it's a simple case of PC inrush exceeding the tripping characteristic of the breaker - a measument of Zs at the busbar end will tell you if swapping to a type D is practicable - if not it's a dialogue with the client to see what action they take in the event of a total or partial power loss - you'd like to think the FM team would be investigating and would generally be on hand following supply restoration to deal with a few minor inrush problems - unplugging a few desks on the "trippy" bars would probably solve it.

It's a balance at the end of the day - does the client want twice as many at least busbars installed to overcome a problem that may only occur once every few years or even less frequently.

Have you been reading Section 444 - you need to get out more

regards

OMS

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 03 February 2013 06:57 PM
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Martynduerden

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Cheers all,

The "trippy" bars don't have faults, as you say it's inrush from older IT, the occupiers are complaining of "power problems" on the floor with mcb's / fuses tripping for fun.

Apparently the original design is c1990 and had 70 seats/4bars in 2008 this was raised to 104 on the same system with the combining of two companies.

The latest plan is to totally rearrange the floor increasing the users to 134, they are quite quick to stress that tripping costs them dear but have limited resilience, I need to ensure that I build in practical resilience where practical and sensible to do so.

The budget is not small!

-------------------------
Regards

Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
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