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Topic Title: Electrocution of Firefighters by PV systems
Topic Summary: Useful product?
Created On: 01 November 2017 02:22 PM
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 01 November 2017 02:22 PM
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kenelmh

Posts: 90
Joined: 17 February 2012

Hello one and all,

I am again looking at a couple of potentially innovative systems regarding the electrical safety of firefighters around PV installations, and wondered what the forum might make of them.

In essence they are both a systems that isolate the PV string wiring (via contactors) from points of aggregation when an emergency 'shut down' signal is sent. The system is aimed at fire emergencies in order that the maximum possible DC shock voltage is limited.

The systems basically isolate the DC distribution sections to limit the DC voltage to 120Vdc. The system is aimed at bonded and non-bonded systems (whether or not one pole is earthed via mains earth) and I note that the risk of shock is greatly reduced in the non-bonded system in any case.

My first query was that if the system is really effective at reducing the effect of a shock to one that would be not at all harmful (certainly as firefighters have a propensity for squirting water over large areas).

Presuming there is a complete path for fault current, the 120Vdc potential appears, to me, to be eminently able to cause a fault current sufficient to cause harm / disorientation, assuming the PV panels were able to supply such. I am also aware that UPS systems do seem to use a similar methodology for limiting the shock potential (internally) to 120Vdc during internal maintenance, for similar reasons.

Are there any thoughts on how effective limiting the voltage to 120Vdc would be?

Or does anyone have experience of these systems?

Thank you in advance.
 01 November 2017 06:11 PM
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Fm

Posts: 1735
Joined: 24 August 2011

My system has been in for 6 years and imagine before the fits were cut lots were installed, unless you get mcs approval for the equipment it isn't going to be a winner
 01 November 2017 08:20 PM
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Dave69

Posts: 621
Joined: 16 July 2011

As a sparks and also retained firefighter im really not sure what you are talking about. All PV panels have IP rated connections so water ingress isnt a real problem, if the mains power is lost the inverters shut down, ok the dc voltage wil, still be present at tne dc isolator, but this should also be ip rated.if you are talking about putting contactor contacts in series with the panels you are just creating BIGGER problems as I can imagine the contactors will be mounted in a loft space so indivual strings will have to run back to their, in a normal PV install its quite simple, on a house maybe a dozen panels and just one string, so if we need to, we just snip some links on the roof because all the panels are wired in series, if we sudenly end up with two or three strings that are series up via contactors we could well end up chopping what would logically be the start and end of one big string only to find the install is split up into smaller strings. Also 120VDC can kill so think you need to look at your figures again.
By the way what do you happens when we enter a property when the mains power is energised and we spary water everywhere?
 02 November 2017 06:32 AM
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AJJewsbury

Posts: 16014
Joined: 13 August 2003

120V d.c. puts it into ELV territory, so that should be safe enough for most situations.

My first question would be how real is the risk in the first place - AFAIK most domestic scale PV systems are floating on the d.c. side (as you observed) and having a few hundred volts between two wires on the roof is a long way from having even a small proportion of that voltage across a vital part of a firefighter. Water itself isn't a particularly good conductor - just look at all the bare HV overhead lines with just a few inches of glass or pot insulators that get covered with water every time it rains or the high resistance of even a single plastic push fit plumbing fitting in a copper water pipe (around 15k Ohms from one experiment of mine) It's wet skin and lack of footwear in wet areas that a problem when it comes to shock rather than water itself. Granted firefighting water will likely get contaminated with carbon deposits etc. which may increase its conductivity a little, but even so I suspect that few metres away from even the bare ends of the d.c. wires you'd be hard pressed get a significant voltage available, let alone with enough current to be a serious hazard.

I understand that in the US (with 115V a.c. mains) firefighters routinely use water on electrical fires - as they don't consider the risk of shock to be that significant.

In your solar farm kind of situation (hundreds or thousands of panels in a field) you may well have higher voltages available and more likely earth referenced, but how likely is it that firefighters would want to pour water on such a system even if it did catch fire?

The other question would be about the 'emergency shut down' signal and who/what would trigger it and how reliable it would be in a building that's on fire.

- Andy.
 02 November 2017 09:43 AM
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kenelmh

Posts: 90
Joined: 17 February 2012

Originally posted by: Dave69

As a sparks and also retained firefighter im really not sure what you are talking about. All PV panels have IP rated connections so water ingress isnt a real problem, if the mains power is lost the inverters shut down, ok the dc voltage wil, still be present at tne dc isolator, but this should also be ip rated.if you are talking about putting contactor contacts in series with the panels you are just creating BIGGER problems as I can imagine the contactors will be mounted in a loft space so indivual strings will have to run back to their, in a normal PV install its quite simple, on a house maybe a dozen panels and just one string, so if we need to, we just snip some links on the roof because all the panels are wired in series, if we sudenly end up with two or three strings that are series up via contactors we could well end up chopping what would logically be the start and end of one big string only to find the install is split up into smaller strings. Also 120VDC can kill so think you need to look at your figures again.

By the way what do you happens when we enter a property when the mains power is energised and we spary water everywhere?


I have to say, I agree with everything you have written. I would add that this system (it's been proposed to me, its not mine) is aimed at larger, rooftop systems where there are significantly more panels. Say 80-300m2 worth where the aggregated voltage can approach 600Vdc, and one line might be earthed. But as you say, more complication, more risk of something not working as intended.

I hope I didn't offend you with the comment about fire fighters squirting water all over the place!
 02 November 2017 09:51 AM
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kenelmh

Posts: 90
Joined: 17 February 2012

Originally posted by: AJJewsbury

120V d.c. puts it into ELV territory, so that should be safe enough for most situations.

.


I don't mean to be a smart-alek but how do we measure safe enough? I tend to agree with Dave69 in that 120Vdc is dangerous, certainly if one line is grounded and you end up with the other exposed (say through the action of a fire).

The issue of fire fighters electrocution due to PV installs appears to be a real issue in that it has caught the attention of the likes of RiscAuthority, BRE and LFB.
 02 November 2017 10:00 AM
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kenelmh

Posts: 90
Joined: 17 February 2012

Originally posted by: Fm

My system has been in for 6 years and imagine before the fits were cut lots were installed, unless you get mcs approval for the equipment it isn't going to be a winner


Doesn't MCS only matter in terms of panel, inverter and the installer?

Thanks
 02 November 2017 10:31 AM
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AJJewsbury

Posts: 16014
Joined: 13 August 2003

I don't mean to be a smart-alek but how do we measure safe enough?

For the likes of me at least, by referring to accepted standards - e.g. BS 7671 (and so all the international standards that's based on) - 120V d.c. falls into the range allowable for PELV (e.g. one pole earthed) as well as SELV - hence no disconnection necessary for protection against electric shock in general (reinforced by the 2nd column of table 41.1 if you're hung up on treating it as ADS). Risk wise 120V d.c. seems to equate to 50V a.c. There are some situations where the limits are reduced to 25V a.c. and 60V d.c. - usually where basic protection is absent or even 12V a.c./30V d.c. - but where basic protection is provided then the higher voltage range seems to be acceptable - even in wet areas like bathrooms.

Maybe Dave could comment on my guess that the firefighter's uniform would normally include insulating footwear and insulating gloves and the overall design is such to keep water out?

- Andy.
 02 November 2017 10:36 AM
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mapj1

Posts: 9577
Joined: 22 July 2004

Actually there are plenty of cases of people being killed by less that 120V on a DC supply - OK for small areas of skin, dry and so forth, but in a situation like a fire, I'd really not like to guarantee that.
Even 28V military vehicle supplies have finished folk off when they already had reduced body resistance because of damage to the outer layers of skin for other reasons, and there have been the odd fatality from defective ELV lighting in swimming pools. Pure rain water is indeed a reasonable insulator - we use distilled as the dielectric in pulse capacitors in some cases, but you only need to stir in a spoon of salt into bucketful to see the resistance plummet. Oddly enough we also use water with carefully controlled concentrations of various ions to make high power load resistors as well, and sometimes even pump the water round to cool it.
The fire brigade don't generally pump de-ionised water- in town it tends to be tap water, or when out in the sticks, whatever they can suck up, then with all sorts of foaming agents added.

I'd suggest that reduced body resistance, both from skin injuries such as burns, and contact with impure water are very credible risks.

I know the 25V touch voltage limit we used to have for high risk situations has been raised to 50v to harmonise it with other folks rules, but the advice was actually in there with good reason.

-------------------------
regards Mike
 02 November 2017 11:39 AM
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broadgage

Posts: 2435
Joined: 07 August 2007

Presumably these proposed contactors will have the coils energised continually under normal conditions ?
Thereby consuming electricity forever, even at night when none is being produced.
Several contactors could consume 10 watts in total, forever. At a cost of perhaps £20 a year or £250 over the life of the installation.
Installation costs would be increased and complications introduced and reliability reduced.

Seems to me to be a solution to a non problem.
 02 November 2017 12:08 PM
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mapj1

Posts: 9577
Joined: 22 July 2004

In terms of dissipation maybe one time explosively blown links would be better - no current to consume until you trigger them.
Even something like fuse-wire holding against a zebedee spring...
Been done apparently

-------------------------
regards Mike
 07 November 2017 11:24 AM
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kenelmh

Posts: 90
Joined: 17 February 2012

Thanks all for the replies, all useful. My thoughts are that the system introduces unnecessary levels of complexity, points of failure and potentially a false sense of security. This against the merit of a reduced shock potential but to a level where we are not sure that it no longer presents a significant hazard.

A better solution in my mind would be micro-inverters but I'm not sure of the economics of that on a large installation. Obviously they should assist in optimizing the output when working, reduces the live string wiring to that between the PV panel and the micro-inverter, and the micro inverter will limit the DC shock potential to much lower levels when AC mains is taken away. Would welcome any comments regarding the relative economies of micro-inverters to traditional 'array' inverters.

Thanks
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