DNO questions

[Steve]

Fortunately there was a powercut at work the other day. We got to shut up shop for 2 hours. When they dug the road up, they found a single phase tee-joint had blown up, causing one phase to cut out. The other 2 phases continued working, much to the distress of our main 3-phase fridge compressor, which tried to carry on working.

We were lucky - the phase that blew out only poweres a couple of fridges and 2 rows of lights. So glad it wasnt the phase that controls all the lighting contactors! This phase was off from 7:30pm to 12 midday, so this would have been disastrous for us!

When the power came back on, it flickered on, a phase at a time. The final underground repair connection was made live Surely there would be a lot of arcing, reconnecting a loaded wire like this (hence the flickering?). How would the power have been cut, so the repair could be carried out in the first place? Or do they just cut the damaged cable out under load? What sort of PPE do these guys have for this sort of work?

Also, I saw the cable they used to make the join underground, it was 3 core (maybe 4 core) with an outer sheath IIRC, and the cores were triangular. What is this cable known as? Would this have been aluminium cores? (its worth a mention here that our shop has a 3 phase TNS supply, though it may be TNCS, its not very clear looking at the cutout)

Any DNO or ex-DNO guys hereabouts?

Just curious!

 

[ColJack]

larger size multicore cables tend to be triangular ( well pie shaped, flat 2 sides, curved the third.).

it saves on wasted space..

it may or may not have been aluminium, it depends on what the original cable was, what voltage it was ( the higher voltage stuff tends to be red with a ridged looking armor on it, ie spiral wound like the metal adaptaflex. )

 

[bernardgreen]

The other 2 phases continued working, much to the distress of our main 3-phase fridge compressor, which tried to carry on working.

A good reason to have a phase failure detector fitted to protect you three phase equipment for damage.

And automatically switch any single phase essentials onto one of the surviving phases.

Of course it will never happen again. Will it........

 

[stevesey]

When the power came back on, it flickered on, a phase at a time. The final underground repair connection was made live Surely there would be a lot of arcing, reconnecting a loaded wire like this (hence the flickering?). How would the power have been cut, so the repair could be carried out in the first place? Or do they just cut the damaged cable out under load? What sort of PPE do these guys have for this sort of work?

My Dad was a DNO engineer/manager - I got to go and sit the car and get bored when he was on call sometimes - also got to see the inside of a few sub-stations and distribution buildings. Memories of massive switch gear and isolators spring to mind.

From what I picked up they'd isolate the cable at the sub-station, possibly re-routing some supplies temporarily to get some consumers back on. Then fix fault and re-connect. I've always assumed flickering shortly after you come back on is the feeds they re-routed being switched back. Also remember being told they earth the cable, as because of capacitance you can still get a fatal shock (this may have been for longer runs of 33Kv+ though).

Our whole estate was supplied by 4 core aluminum cable (3 + neutral). Apparently they used a lot of a certain type of this in the early 80's - until they found out the neutrals would burn out over time. We used to get lots of power cuts and had several holes in our road. They replaced the whole lot (1000 houses) about 5 years ago. One went outside us once - guess how they pinpointed the fault. Engineer drills about a 8mm hole in pavement and sniffs

 

[Lectrician]

When working with HV they always earth down the cores they are working on.

LV they dont.

They would isolate at the sub, and would indeed re-route other supplies if parallel networking was available - basically dead lengths of cable that need a couple fuses putting in and taking out to power areas from a different sub or circuit.

They would have cut the cable at the fault, pot ended it, re-instated power, and then jointed the cables back together.

The cable would more than likely be ALU if it was in the street and not coming to the property - generally copper to the properties on smaller supplies. It would probably be a waveform concentric cable, being 3 or 4 core depending on if it was TN-C-S or TN-S. more than likely 4 core - they tend to install this these days even for PME.

Triangular cores are common in 3/4 core cables at larger sizes. 2 core would be semi-circular.

 

[dannyboi2003]

Hi Crafty,

We had a minor/major power failure at work last Monday. We were having an additional air conditioning unit installed into our server room and we arranged for a contractor to install the mains supply. The supply was taken from the board that supplies all power to the server room and we told the contractor that they could under no circumstances disconnect the mains power to that board. There are several spare mcb's so was an easy enough job.

All was going well until my email went down and we started getting phone calls asking if the network was down, popped my head in the server room and all the UPS's were beeping like crazy, and some had already died. Told the contractor who found that the 60amp breaker for the board (located in the switch room) had tripped - but it had nothing to do with him apparently

By the time power had been restored most of the UPS's had died and all eight of our sites were offline, no phones, web or email. Could have been worse as total down time was only about 15 mins, once the core switch was back up everything was fine.

That was frustrating enough so can only imagine how it must have been for you.

All the best
Dan

 

[RF Lighting]

Also, I saw the cable they used to make the join underground, it was 3 core (maybe 4 core) with an outer sheath IIRC, and the cores were triangular.

Triangular SWA

 

[Spark123]

Can't be very large UPS systems if they died so soon?

Can you bend that round ya little finger Rob?

 

[RF Lighting]

Can you bend that round ya little finger Rob?

Easy. It's only 35mm² 4 core.

 

[dannyboi2003]

Hi Spark123,

The 6 data cabinets each have a 3000va APC ups in the bottom which feeds all the equipment in each cabinet. Theres a 1500va APC ups on the phone system, a 1000va APC ups on the incoming data links cabinet (BT/NTL etc.) There's a few 1000/1500 ups's powering some smaller items that are not rack-mountable and therefore sit on some steel shelves.

Financial funds are not available to install anything better, as long as we have surge protection and battery back up to get us through brief outages thats the main thing. The first UPS dies after about 10 mins and the only reason so many died this time was that we had no idea that the power had failed until the network went down, also locating the main breaker for the board took a while too.

Fortunately the phone system stayed up as its a right pain in the a**e if it goes off without being powered down properly.

All the best
Dan

 

[Spark123]

You don't get very long on them then? Are they only designed to run till a generator kicks in or something? No alarm outputs on the UPS units? The UPS systems I used to get as an apprentice, each cell was about 1ft x 2ft x 2ft! These consisted of a charger, battery, inverter which was on line feeding the load all the time. Appos job was to fill each cell up with distilled water, volt and sg check each one. Was hundreds of the blinking things to do too.

 

[dannyboi2003]

Nah no generators, way beyond what we could afford. APC ups units are just standard off the shelf units anyone can buy, they do give an audible alarm when the powers failed but with the server room door closed its not easy to hear them, plus we are another room away too.

lol, you were kept kinda busy then.

All the best
Dan

 

[securespark]

Rob, is there anything you haven't got a picture of????

 

[RF Lighting]

I had to go outside in the rain to take that photo. It was on my scrap pile.

I have about 1500 photos in my archive to date

 

[Lectrician]

APC ups's have serial and USB interfaces to connect to a server. You can connect as many as you want to a network, and use one server to monitor the lot. As soon as one fails it sends a broadcast message over the network causing an instant popup on the screen of any computers in the same workgroup. You can also fit a relay card to any server designed to respond to this broadcast message and take appropriate action, such as sounding an alarm, sending a message via an autodialler etc.

You can also configure each UPS to send a shut down signal to the server at mains failure, or just before the battery goes into a critical level.

The APC range come with the software in the box, or it can be downloaded for free. They also supply a serial and a USB cable. No excuse for not using it! Just connect each APC to the server it is protecting, and then run the maintenance console on your PC in the office - you can then check the status of all APC's via the fixed IP of each server. It records all outages, brownouts etc. You can check battery level, load level remotely.

All, as you say, out of the box - I would go an configure it all properly if I where you!