Safely disconnect battery string

[ampsie]

Hi All,

I'm moving into telecoms and looking for some help on disconnecting battery strings.
I come from a domestic background so have not got involved in battery strings before as I never touched solar.

My questions is thus:
How do you safely disconnect a battery string that is housed in a tight earthed/grounded rack? If there is minimal space between the battery terminal and the earthed metal of the rack.

I have been advised to disconnect the negative first on a short course I have undertaken but if you were to touch the cable to the rack whilst it was still on the battery terminal this would cause a short wouldn't it?
Am I correct in assuming this would not be the case if the fuses have been pulled and the negative cable has been completely removed from the negative battery terminal?

I apologise if this is a rookie question - just looking to arm myself with as much information as I can prior to entering the industry.

TIA

 

[R-fur]

We need a bit more information.
Is the battery earthed? You say its for telecoms, which traditionally had a positive earth.
Is the metal stand earthed? it may be isolated.
Disconnecting the negative first is correct on cars but on higher voltage battery's it make sense to remove an easily accessible link near the midpoint. this should mean that you are working at a safer voltage and immediately halves the fault level.
The company you are working for should have risk assessed battery works and produced method statements for doing standard jobs.
BE VERY CAREFULL big battery's are dangerous and have hazards that you might not fully appreciate coming from domestic

 

[ampsie]

We need a bit more information.
Is the battery earthed? You say its for telecoms, which traditionally had a positive earth.
Is the metal stand earthed? it may be isolated.
Disconnecting the negative first is correct on cars but on higher voltage battery's it make sense to remove an easily accessible link near the midpoint. this should mean that you are working at a safer voltage and immediately halves the fault level.
The company you are working for should have risk assessed battery works and produced method statements for doing standard jobs.
BE VERY CAREFULL big battery's are dangerous and have hazards that you might not fully appreciate coming from domestic

Yes, so it would be a positive earth.
The racks in which the batteries are housed are earthed so there is voltage between the battery terminals and the rack itself.

Yeah, I'm aware of the potential for danger which is why I'm keen to understand the best practice for doing this safely.

Would removing links be safer? It would not produce arcing removing links from the centre of the string?

 

[Simon47]

What sort of battery size are you thinking about ? It makes a big difference.
Small batteries with insulated faston terminals aren't too much of a problem - but again I'd suggest following the earth first off, earth last on described below. But as the batteries get bigger, the risks get higher. In the past I've worked with big batteries - I'm no lightweight, when one of the cells would have needed several of me to tip the scales in my favour - you definitely treat those with respect.

The bit about disconnecting negative first possibly comes from the automotive world where vehicles have been negative earth for many decades. The idea is, if you short the negative terminal to earth while undoing it (e.g. you put a spanner between terminal and bodywork) then there is no voltage (you are just going in parallel with the earth strap) and it won't cause a spark. Once the negative terminal is off, then you can safely remove the positive terminal as there's no circuit if you touch the spanner on anything (other than the battery negative which is generally hard to do on an automotive battery). Refitting is the reverse - make the earth terminal last.
Applying the same logic to a positive earth battery would mean undoing the positive terminal first - on the basis that it won't make sparks if you short the spanner to chassis. After that, you've broken the circuit for shorting another terminal to chassis and making sparks.

But then you get into all sorts of questions.
What system voltage is present ? Depending on the battery voltage, you may well get to the stage where some terminals are not safe to touch - either while touching something earthed or another battery terminal.
Is battery "earth" the same as the enclosure "earth" - that can't be assumed in all systems ?
And if it is, is that an easy terminal to get to ?
Have you got suitable tools - in the naval world they have fully insulated tools for working on big batteries, and I assume other industries will have similar. Dropping something non-insulated across the terminals of something like a submarine or BT phone exchange battery will (at the very least) mean that's the last you see of the tool. I recall a friend who worked for BT at the time telling me how a contractor dropped a pry-bar (what I'd call a jemmy) across the busbars in an exchange. It sparked as it connected, got too hot to grab and pull off before he could get to it, started glowing ... red ... yellow ... and finally dripped on the floor. For good measure, BT charged him (or his employer) for recharging the battery !

And then some systems are just "poorly designed". At a previous employer, we bought a UPS that used two strings of 10off 12V blocks, and IIRC 110AHr - i.e. 120V 220AHr nominal in total. The cells fitted onto several levels in a cabinet, and you had to start at the bottom, fit one connecting cable (which went to an isolator at the top of the cabinet) and some blocks, the inter-block links, and the link to the next block on the next layer. Then you fitted a metal tray on it's supports, and completed the first string - so now there was 120V across the battery side of the isolator.
You then fitted another tray before starting the second string. But, even though they had used a 3 pole isolator, they only used 2 poles of it. So you had to undo the bolt in one terminal, fit the end cable for the second string to it, and refit it - then build up the second string in the same way. Finally, you have a free cable which is now at 120V DC (nominal) relative to other bits, and have to fit it onto the same bolt as another cable that comes up from the depths of the cabinet. I "wasn't impressed" with that design - who designs a system that forces you to be waving a free lead about, and indeed grapple with two of them, when you've 120V just waiting to make your day rather memorable ?

 

[R-fur]

Yes I should of said pull the fuses or open the links before trying to disconnect terminals. What Simon describes is why I break the mid point first and make it last, unfortunately it sounds like he did not have the room to do that. I have even heard of system which have a mid point fuse so that if a spanner is dropped across terminals then the mid point fuse may blow and reduce the damage.
For what its worth, most of my battery work is done in grid substations with 120V battery's made up of two volt cells, each cell is often a two man lift and the battery's are housed in custom built rooms. Normally our battery's run earth free, with an earth fault monitoring system that alarms if there is a positive or negative earth fault.

 

[ampsie]

What sort of battery size are you thinking about ? It makes a big difference.
Small batteries with insulated faston terminals aren't too much of a problem - but again I'd suggest following the earth first off, earth last on described below. But as the batteries get bigger, the risks get higher. In the past I've worked with big batteries - I'm no lightweight, when one of the cells would have needed several of me to tip the scales in my favour - you definitely treat those with respect.

The bit about disconnecting negative first possibly comes from the automotive world where vehicles have been negative earth for many decades. The idea is, if you short the negative terminal to earth while undoing it (e.g. you put a spanner between terminal and bodywork) then there is no voltage (you are just going in parallel with the earth strap) and it won't cause a spark. Once the negative terminal is off, then you can safely remove the positive terminal as there's no circuit if you touch the spanner on anything (other than the battery negative which is generally hard to do on an automotive battery). Refitting is the reverse - make the earth terminal last.
Applying the same logic to a positive earth battery would mean undoing the positive terminal first - on the basis that it won't make sparks if you short the spanner to chassis. After that, you've broken the circuit for shorting another terminal to chassis and making sparks.

But then you get into all sorts of questions.
What system voltage is present ? Depending on the battery voltage, you may well get to the stage where some terminals are not safe to touch - either while touching something earthed or another battery terminal.
Is battery "earth" the same as the enclosure "earth" - that can't be assumed in all systems ?
And if it is, is that an easy terminal to get to ?
Have you got suitable tools - in the naval world they have fully insulated tools for working on big batteries, and I assume other industries will have similar. Dropping something non-insulated across the terminals of something like a submarine or BT phone exchange battery will (at the very least) mean that's the last you see of the tool. I recall a friend who worked for BT at the time telling me how a contractor dropped a pry-bar (what I'd call a jemmy) across the busbars in an exchange. It sparked as it connected, got too hot to grab and pull off before he could get to it, started glowing ... red ... yellow ... and finally dripped on the floor. For good measure, BT charged him (or his employer) for recharging the battery !

And then some systems are just "poorly designed". At a previous employer, we bought a UPS that used two strings of 10off 12V blocks, and IIRC 110AHr - i.e. 120V 220AHr nominal in total. The cells fitted onto several levels in a cabinet, and you had to start at the bottom, fit one connecting cable (which went to an isolator at the top of the cabinet) and some blocks, the inter-block links, and the link to the next block on the next layer. Then you fitted a metal tray on it's supports, and completed the first string - so now there was 120V across the battery side of the isolator.
You then fitted another tray before starting the second string. But, even though they had used a 3 pole isolator, they only used 2 poles of it. So you had to undo the bolt in one terminal, fit the end cable for the second string to it, and refit it - then build up the second string in the same way. Finally, you have a free cable which is now at 120V DC (nominal) relative to other bits, and have to fit it onto the same bolt as another cable that comes up from the depths of the cabinet. I "wasn't impressed" with that design - who designs a system that forces you to be waving a free lead about, and indeed grapple with two of them, when you've 120V just waiting to make your day rather memorable ?

They would typically be 12v batteries wired in series to have a voltage of 48v so is theoretically a touch safe voltage. Depending on the system there'd be a number of these strings wired in parallel depending on the needs.

The rack earth would share the same earth as the batteries I would assume as the earth for the rack will typically come from the busbar.

Yes, fully insulated tools.

From what I have seen from the course I did, the racks are always earthed and the batteries sit within these racks but with the end terminals very close to the metal of the rack. The conductors have metal lug crimps on which I worry can come into contact with the metal of the rack and the terminal of the battery at the same time during disconnection even if insulated tools are being used.

So, in effect the safest way to isolate would be to disconnect all fuses, identify the 0v conductor (whether that is a positive or negative earth) and remove this first? Would this conductor still short if it came into contact with the rack once disconnected or would it be completely safe?

 

[Simon47]

Ideally the battery should have double-pole isolation - that way, once isolated it becomes a floating system. As a floating system, you can't create current by shorting any single terminal to earth or the case. To create a current (and hence a spark) you'd need to short between two battery terminals or have two terminal-earth shorts at the same time. If batteries are in parallel, then ideally you should have a means of isolating each parallel string - though I suspect it's not that common.

If you can't isolate the battery (that's a bad design, but I'm sure there's plenty of system like that), then yes - disconnect the "earthy" terminal first as that minimises the risks from shorting it to earth (e.g. with the spanner) while undoing it. However, that might not make the non-earthy terminal (or more importantly, the loose bare-ended wire you've just removed from it) much safer - especially if you touch another battery terminal with it.