Victron BMV 712 - Does it have a memory?

[cuthound]

12 minutes ago, ivan&alice said:

I have four 400W panels in series so they produce up to 10A at 160V. I'm not sure what the threshold of danger is but that seems like enough to cause a pretty big spark if I disconnect them abruptly on a sunny day - possibly even a shock if I touch the wrong things?

 

Seems to me like a breaker is a good idea, I don't care much whether it is a BSS requirement - it just feels like it would make maintenance tasks (like switching the polarity of shunts) a lot easier. Ideally I'd like to have breakers everywhere really but the low voltages on most of the system make that non cost effective. Since you can get a 10A DC breaker fairly affordably it seems like a no brainer to add one to the solar input!

 

No one answered my other question so I assume that it's not a problem to have solar panels in series in the sun not connected to anything, i.e. if you have the breaker turned off`? What happens to the energy - would they simply get hot if they didn't have any load on them?

 

When I was working most UPSs had 400 volt batteries. They were kept in an interlocked room. When you opened the door to gain access to the batteries, the interlocks automatically split the battery into 50 volt sections to meet the requirements of "extra low voltage", 50 volts DC being the highest DC voltage still categorised as ELV in the IET regs (which dont apply on boats but many aspects of which relate to good practice wherever the installation is).

 

With regard to your second question, nothing happens to the energy when the breaker is open, as the panel is a power source and until a circuit is formed it is potential energy. Exactly the same as a 13 amp socket without anything plugged into it.

Edited April 23, 2020 by cuthound
To add the last paragraph

[David Mack]

With such dc voltages is it permitted/sensible to run solar panel to controller wires in the same duct/trunking as 12V dc wiring?

[Richard10002]

2 hours ago, WotEver said:

No doubt. But that’s a world away from an (ex) BSS examiner asserting that it’s mandatory. 

I think that's what I intimated..... but somebody above asked about practical use and, for anybody reading the thread in the future, it seemed worth adding in my practical use.

 

nicknorman: "what in practical benefit arises from having an isolator between panels and controller for systems where the panel voltage isn’t dangerous,"

Edited April 23, 2020 by Richard10002

[Richard10002]

31 minutes ago, ivan&alice said:

Seems to me like a breaker is a good idea, I don't care much whether it is a BSS requirement

Those were my thoughts, and worked out in practice.... there is obviously an argument for having as few connections as possible, for a variety of reasons, so DC breakers all over the place is possibly not a good idea, unless necessary in your circumstances.

 

[Keith M]

34 minutes ago, ivan&alice said:

I have four 400W panels in series so they produce up to 10A at 160V. I'm not sure what the threshold of danger is but that seems like enough to cause a pretty big spark if I disconnect them abruptly on a sunny day - possibly even a shock if I touch the wrong things?

 

Seems to me like a breaker is a good idea, I don't care much whether it is a BSS requirement - it just feels like it would make maintenance tasks (like switching the polarity of shunts) a lot easier. Ideally I'd like to have breakers everywhere really but the low voltages on most of the system make that non cost effective. Since you can get a 10A DC breaker fairly affordably it seems like a no brainer to add one to the solar input!

 

No one answered my other question so I assume that it's not a problem to have solar panels in series in the sun not connected to anything, i.e. if you have the breaker turned off`? What happens to the energy - would they simply get hot if they didn't have any load on them?

With that level of voltage a double poled isolator would be a better way to address the situation.

If you do use a circuit breaker ensure it is rated for DC voltage up to about 200 volts. 

[jetzi]

11 minutes ago, Richard10002 said:

there is obviously an argument for having as few connections as possible, for a variety of reasons, so DC breakers all over the place is possibly not a good idea

I think I'd rather have a formal connection that I make once and make well, with a switch, than having to unscrew and rescrew the connection into my MPPT every time I want to disconnect it. Perhaps "everywhere" was a bit strong, but I'd like to be able to isolate major components of the system easily for maintenance and emergencies.

 

8 minutes ago, Keith M said:

With that level of voltage a double poled isolator would be a better way to address the situation.

If you do use a circuit breaker ensure it is rated for DC voltage up to about 200 volts. 

Isn't the rating more to do with the amps than the volts? Hence why you can have relatively small breakers for AC mains compared to DC isolators? Why is a double poled isolator better than a breaker?

I'd want a breaker/isolator that could handle up to 250V, which is the maximum input of my MPPT, in case I decide to add another two panels one day (making 6x 40VOC = 240V => 24A max).

 

So, no dump load required for solar - they are safe to leave in the sun connected to nothing?

Edited April 23, 2020 by ivan&alice

[cuthound]

10 minutes ago, ivan&alice said:

I think I'd rather have a formal connection that I make once and make well, with a switch, than having to unscrew and rescrew the connection into my MPPT every time I want to disconnect it. Perhaps "everywhere" was a bit strong, but I'd like to be able to isolate major components of the system easily for maintenance and emergencies.

 

Isn't the rating more to do with the amps than the volts? Hence why you can have relatively small breakers for AC mains compared to DC isolators? Why is a double poled isolator better than a breaker?

I'd want a breaker/isolator that could handle up to 250V, which is the maximum input of my MPPT, in case I decide to add another two panels one day (making 6x 40VOC = 240V => 24A max).

 

So, no dump load required for solar - they are safe to leave in the sun connected to nothing?

 

No, a DC current is much more onerous on the breakers contacts than an AC one, because the AC current falls to zero 50 times a second, whereas the DC current remains constant.

 

An isolator is designed to open a circuit without load on it, whereas a circuit breaker is designed to break a circuit under load, therefore it should last longer.  The double pole part is to prevent the voltage backfeeding via the negative or neutral pole snd giving you an unwelcome shock.

Edited April 23, 2020 by cuthound
To add the last paragraph

[cuthound]

57 minutes ago, David Mack said:

With such dc voltages is it permitted/sensible to run solar panel to controller wires in the same duct/trunking as 12V dc wiring?

 

Possibly, mixing DC and even DC & AC cables at different voltages in the same containment is permissable under BS7671, providing the voltages are in the same voltage band and the insulation of all cables is rated for the highest voltage present.

 

In the late 1990's BS7671 introduced the concept of voltsge bands to simplify matters, and allowed AC and DC voltages in the same band to share the same containment.

 

Voltage Band I is defined as levels of voltage which are too low to provide serious electric shocks; effectively this limits the band to extra-low voltage (ELV), including telecommunications, signalling, bell, control and alarm circuits.

Voltage Band II covers all voltages used in electrical installations not included in Band I. This means that all 230/400 V (240/415 V) supplies are included in Band II.

As expected, BS 7671 prohibits Band I and Band II cables sharing the same cable enclosure or multicore cable unless: every cable is insulated for the highest voltage present,

Edited April 23, 2020 by cuthound
Clarification

[WotEver]

1 hour ago, ivan&alice said:

I assume that it's not a problem to have solar panels in series in the sun not connected to anything, i.e. if you have the breaker turned off`?

That’s correct. 

[WotEver]

1 hour ago, ivan&alice said:

I have four 400W panels in series so they produce up to 10A at 160V. I'm not sure what the threshold of danger is but that seems like enough to cause a pretty big spark if I disconnect them abruptly on a sunny day - possibly even a shock if I touch the wrong things?

 

Seems to me like a breaker is a good idea,

For that voltage, absolutely. A breaker would be strongly advisable. 

Finding a D.C. breaker to suit that spec might be an interesting exercise. 

45 minutes ago, ivan&alice said:

So, no dump load required for solar - they are safe to leave in the sun connected to nothing?

Yes. 

[nicknorman]

1 hour ago, ivan&alice said:

I think I'd rather have a formal connection that I make once and make well, with a switch, than having to unscrew and rescrew the connection into my MPPT every time I want to disconnect it. Perhaps "everywhere" was a bit strong, but I'd like to be able to isolate major components of the system easily for maintenance and emergencies.

 

Isn't the rating more to do with the amps than the volts? Hence why you can have relatively small breakers for AC mains compared to DC isolators? Why is a double poled isolator better than a breaker?

I'd want a breaker/isolator that could handle up to 250V, which is the maximum input of my MPPT, in case I decide to add another two panels one day (making 6x 40VOC = 240V => 24A max).

 

So, no dump load required for solar - they are safe to leave in the sun connected to nothing?

Can your controller actually tolerate 240v? But anyway, as I said earlier, with those sorts of voltages an isolator (which is not the same as an overcurrent circuit breaker) is obviously a good idea. The rating is about both volts and amps, for different reasons (amps for resistance / heating / arcing, volts to ensure the contacts are far enough apart when open).

 

As has been mentioned DC can be hard on breakers, but set against that the inductance of the wiring between panels and controller, and the controller itself, is minimal so relatively little propensity to form an arc.

Edited April 23, 2020 by nicknorman

[Richard10002]

1 hour ago, WotEver said:

For that voltage, absolutely. A breaker would be strongly advisable. 

Finding a D.C. breaker to suit that spec might be an interesting exercise. 

Yes. 

Bimble have 80A, 100A, and 125A DC breakers with a 250V limit.

 

https://www.bimblesolar.com/extras/dc-circuit-brakers/80a-din-breaker-tomzn

[WotEver]

11 minutes ago, Richard10002 said:

Bimble have 80A, 100A, and 125A DC breakers with a 250V limit.

 

https://www.bimblesolar.com/extras/dc-circuit-brakers/80a-din-breaker-tomzn

Well, that’s simple then

[PeterF]

You can get specific double pole solar isolators for high DC voltages such as https://www.ebay.co.uk/itm/32A-2-POLE-4-POLE-1000VDC-TRUE-DC-ENCLOSED-ROTARY-ISOLATOR-IP66-SOLAR-PV/172752383221 or https://www.ebay.co.uk/itm/Solar-Panel-Isolator-SALZER-D221-83200-700N1-32A-450V-to-20A-650V/121912721002

 

However, they are quite large..

Edited April 23, 2020 by PeterF

[jetzi]

2 hours ago, cuthound said:

No, a DC current is much more onerous on the breakers contacts than an AC one, because the AC current falls to zero 50 times a second, whereas the DC current remains constant.

This makes complete sense, thanks!

 

2 hours ago, cuthound said:

An isolator is designed to open a circuit without load on it, whereas a circuit breaker is designed to break a circuit under load, therefore it should last longer.  The double pole part is to prevent the voltage backfeeding via the negative or neutral pole snd giving you an unwelcome shock.

Yes so I want a circuit breaker, to break the circuit while under load (sun shining). I can't say I understand what voltage backfeeding could look like or how it could prevent shocks. Is there one input and two outputs in a double pole isolator?

 

1 hour ago, nicknorman said:

Can your controller actually tolerate 240v?

I have a Victron SmartSolar 250/100 - I splashed out and overspecced in case I want to add more generation capacity in the future. According to the datasheet it tolerates 245V start up and 250V absolute maximum. 6 of my panels at 40.48VOC in series would be 242.88V. That may well be toeing the line far too closely, but if that's the case I can always switch to to 3s2p at 120V or even 4s2p at 160V.

 

Honestly I am absolutely blown away by how much power I'm getting out of them. The worse we've harvested, on a rainy overcast day last week, was 500Wh and it's usually closer to 950Wh. We only need to run our engine now for hot water and for cruising. Granted, it is nearly May but I'm well pleased with the result.

 

1 hour ago, nicknorman said:

But anyway, as I said earlier, with those sorts of voltages an isolator (which is not the same as an overcurrent circuit breaker) is obviously a good idea. The rating is about both volts and amps, for different reasons (amps for resistance / heating / arcing, volts to ensure the contacts are far enough apart when open).

 

As has been mentioned DC can be hard on breakers, but set against that the inductance of the wiring between panels and controller, and the controller itself, is minimal so relatively little propensity to form an arc.

I can't say I understand all the physics of what you have said here but I'm hearing that a circuit breaker on my solar input is a good idea, so I will be adding one

 

46 minutes ago, Richard10002 said:

Bimble have 80A, 100A, and 125A DC breakers with a 250V limit.

 

https://www.bimblesolar.com/extras/dc-circuit-brakers/80a-din-breaker-tomzn

Thanks for the link!

 

And thanks @WotEver for confirming that solar can be "left hanging" without a load.

 

[cuthound]

4 minutes ago, ivan&alice said:

I can't say I understand what voltage backfeeding could look like or how it could prevent shocks. Is there one input and two outputs in a double pole isolator?

 

Voltage backfeeding is where a voltage applied to one end of a cable can be measured (or felt if you touch the cable) at the other end.

 

A normal circuit breaker (single pole) only breaks one part of the circuit (the positive or live side).

 

A double pole circuit breaker breaks both sides of the circuit (positive/live and negative/neutral) thus preventing any voltage backfeeding up the negative or neutral side of the circuit and giving you a shock, which can happen with a single pile circuit breaker even though you had opened it and thought the circuit was safe to work on.

[jetzi]

7 minutes ago, cuthound said:

 

Voltage backfeeding is where a voltage applied to one end of a cable can be measured (or felt if you touch the cable) at the other end.

 

A normal circuit breaker (single pole) only breaks one part of the circuit (the positive or live side).

 

A double pole circuit breaker breaks both sides of the circuit (positive/live and negative/neutral) thus preventing any voltage backfeeding up the negative or neutral side of the circuit and giving you a shock, which can happen with a single pile circuit breaker even though you had opened it and thought the circuit was safe to work on.

Ok interesting - so does this mean you would connect 4 wires to it - positive and negative PV and positive and negative MPPT?

[cuthound]

19 minutes ago, ivan&alice said:

Ok interesting - so does this mean you would connect 4 wires to it - positive and negative PV and positive and negative MPPT?

 

Yes, double pole breakers have four connections (positive in & out and negative in & out) but the double pole breaker breaks both the positive and negative circuits or poles rather than just the positive. Single pole breakers can have two or four connections, depending on type and rating.

 

It will go between the PV panel.and the MPPT controller.

Edited April 23, 2020 by cuthound
Clarification