IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Technical Safety Analysis
Topic Summary:
Created On: 17 June 2013 08:56 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 17 June 2013 08:56 PM
User is offline View Users Profile Print this message



earthing

Posts: 56
Joined: 17 December 2008







Hi,

I have just started working for a company installing solar panels. Lets just suffice to say that the previous electrician has left a trail of destruction.

I want to describe exactly what I found today and ask some questions which I hope someone has the answer for!

Firstly, the customer came downstairs to a smoking fuse board. She called and I advised her to isolate her solar installation both d.c and a.c

When I arrived I disconnected the fuse board and discovered that the terminated copper conductor had missed the cage/clamp and must have been loose arcing etc etc.

So first question: Should the fuse have operated?

My understanding is with arcing the current rises thus causing heat and eventually fire. The mcb pictured is the fuse responsible for connecting the inverter to the grid.

As I am sure you will have picked up on, the electrician has also wired a 2.5mm Twin & Earth directly off a 100amp isolator.

Apart from being a potential fire risk this is where my second question is.

With this installation, there were four consumer units. One for sockets, one for lights, one for a shower and one for a garage. The boards were all wired back to Henley blocks.

The customer complained that she was not seeing much savings on her electricity bill despite managing her usage to heavy loads during the day and avoiding heavy use at night.

My question here is concerning metering and the path of the current flow. Is it possible that the customer is not saving as much as she should be? So in the above case only saving on her lighting because thats what the solar array is connected to?

I have always taken separate supplies from the mains via 16/25mm tails. I have never really understood how you can be able to generate and export via the same cable. I understand that a.c flows both ways 50 times per second and that the inverter syncs with the frequency but if someone has some way of explaining this scenario I would sleep better tonight. Just dont understand exactly what is happening here.

Im finding this one difficult to articulate but each CU will draw what it requires from the grid so doesnt this work both ways and the customer will only save on the CU connected with the array.

Can anyone explain that with ohms law or something!!!???

Thanks for your help!
 17 June 2013 09:09 PM
User is online View Users Profile Print this message



daveparry1

Posts: 6106
Joined: 04 July 2007

If something is arcing it's usualy due to a loose connection therefore causing a high resistance so it can't cause the current to increase! It will of course generate heat though,

Dave.
 17 June 2013 09:25 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3459
Joined: 22 November 2007

I'm not an expert on PV but we interface with it a lot on our farm sites.

So here goes, Providing the output from the inverter is set up correctly the load should see inverter supply before grid. It's been a long while since principles at college but I think it was Thevenin's (or similar) theory that covers it. (it's one of them anyway that says the load will draw current from the source with the higher output in a parallel supply situation.

Providing all of the consumer units are henley blocked together on the load side of the meter then they should all see the inverter output equally.

Stu
 17 June 2013 09:28 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 521
Joined: 14 September 2010

be carfeul,

electric arcs are non-ohmic and as such exhibit negative impedence; current will sky-rocket as the voltage collapses; this is why arcs are so destructive. The huge currents in an electric arc can raise local temperatures to 20,000 degrees or higher, ionizing the air and these temperatures are hot enough to destroy even the most robust equipment if not extinguished - with often catastrophic and fatal results.


See the terrible consequences of an out of control arc in a tap changer at Chelmsford Grid which destroyed the sub station and killed the engineer
http://www.hse.gov.uk/press/2013/rnn-e-00213.htm
 17 June 2013 09:48 PM
User is offline View Users Profile Print this message



earthing

Posts: 56
Joined: 17 December 2008

Hi Zuiko,

Thanks for the post that tragic and a potent reminder of how lethal electricity can be.

Just wondering if you thought the mcb should have operated or did your answer explain that and I didnt get it?

You said its non ohmic, negative impedance and the current rises massively. Should this result in the mcb operating?

The scary thing is this is the second call out since saturday with exactly the same thing scenario and ive only been here two weeks.

I am seriously concerned that someones house is going to burn down.

Thanks for your reply
 17 June 2013 09:55 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3459
Joined: 22 November 2007

The difference between the arcs is that yours I think is conductor to MCB terminal and didn't manage to throw molten gases across to another conductor (neutral or earth) which is when the fault currents start to flow and build as zuiko has explained.

I'd be very concerned about your predecessors work and hope your employers are well covered ;-)


Stu
 17 June 2013 10:09 PM
User is offline View Users Profile Print this message



earthing

Posts: 56
Joined: 17 December 2008

Hi Stu thanks for your reply,

Do you think then that its possible that the customer is loosing out on potential savings?

If the load draws from the biggest power source then does that mean that it will take from the grid first?

I must say that the customer is fiercely intelligent and I do feel that shes probably on to something when she says theres something amiss. Im hoping we can get to the bottom of this and fortunately she has been keeping an accurate record of exactly what units per month for a period of years.

The experience thats made me question how the current flows is because I was actually on my way out to the customer to install a solic immersion control.

If you havent experienced/seen/read these before they are simple devices designed to monitor whether the installation is drawing or supplying the grid with surplus power from your solar array.

Where the install is supplying back to the grid the solic diverts this surplus to the immersion heater which saves the customer money. According to the manufacturer Earthwise this system pays for itself in three years I beleive.

Anyway referring back to my earlier explanation of how the four CU's were connected and where the solar array was connected back to, the Solic's current transformer monitoring sensor was installed on the line conductor of the sockets and immersion heater board.

The immersion circuit in to the controller and then from the immersion fuse in to the controller. (Immersion re-routed through the controller and fed from the old fuse)

However the immersion control failed to see any power going back to the grid. This was because the solar array a.c connection was on the opposite lighting board.

This was proved because I rewired the solar array straight through the Henley blocks and reconnected the c.t to the incoming double insulated tail from the import/export meter.

The controller then could see the power from the array.

Sorry if ive lost you here im finding this difficult to articulate but my understanding was that no matter where the solar array was connected the load sensor of the immersion controller should have been able to seen the power from the array.

This is why I am now questioning whether the other three CU's might be able to draw power straight from the grid without utilising the potential saving from the solar array.
 17 June 2013 10:12 PM
User is offline View Users Profile Print this message



earthing

Posts: 56
Joined: 17 December 2008

Thanks Stu. The firm I work for subcontracts the installation from another company so I feel all of our jobs are on the line too!

Sometimes feel that I need an electrical engineering degree and some sort of laboratory to decipher these problems but thats not a luxury I have!
 17 June 2013 10:14 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3459
Joined: 22 November 2007

If I read it correctly, the solic's CT was first around a board that didn't have the PV connected to it ?? ie the sockets and imersion heater with the PV connected to the lighting board ??

If so, it's not going to see reverse current flow because there isn't any at that point, whether the energy is coming from grid or solar into the socket board it's flowing forward.

You've proved this by moving the ct onto a different set of tails (draw it out with the four db's and the flow directions and it will be very clear)

I still go back to questioning the inverter settings as if memory serves from an issue we had a few months ago with export and on site useage, the inverter output is normally set a few volts higher than the nominal grid....... Someone may be able to confirm this that actually does PV


Stu
 17 June 2013 10:27 PM
User is offline View Users Profile Print this message



John Peckham

Posts: 7375
Joined: 23 April 2005

A good starting point would be to carry out a periodic inspection and test of the installation. Part of the I&T would be to look for compliance with Chapter 712 for the PV install. No doubt whoever installed the PV has issued and installation certificate?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 17 June 2013 10:41 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3459
Joined: 22 November 2007

Pull the other one John, it's got bells on it
 17 June 2013 10:53 PM
User is offline View Users Profile Print this message



earthing

Posts: 56
Joined: 17 December 2008

Hi John yes there was a certificate and after testing I beleive the results were recorded from testing and not made up. The ZS was higher than expected and should have caused concern although inside the requirements of the regs. Its a straight through cable run with very little connections. (Not like a ring main or lighting circuit for example) Aside from the identified and rectified problems of terminations, fuse protection for the cable and discrimination I havent identified any other problems.

There is no d.c to test and inspect as the inverters are microinverters and the cable I.R has not been compromised on the main cable run connecting the grid and solar array. I removed the destroyed length of twin and earth completely and re-installed meter tails directly back to the henley blocks.

That mcg equipment is the worst quality equipment I have ever come across and the cage clamp is very badly designed. I can see how it could be very easily missed down the side of the cage clamp, although I suspect a good tug test may have identified any termination problems. The units are pretty much disposable as when you open them back up you destroy the screw holders with no simple way to improvise the covers fixing back on.
 17 June 2013 11:06 PM
User is offline View Users Profile Print this message


Avatar for sparkingchip.
sparkingchip

Posts: 5823
Joined: 18 January 2003

"the inverter output is normally set a few volts higher than the nominal grid..."

Come on chaps, that's what it is all about!
 18 June 2013 08:03 AM
User is offline View Users Profile Print this message


Avatar for dickllewellyn.
dickllewellyn

Posts: 1150
Joined: 19 March 2010

Micro inverters. Is that the little ones on the backs of panels? Does that mean long cable runs from mains position to inverters? Could volt drop at this stage be causing a problem?

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 18 June 2013 01:23 PM
User is offline View Users Profile Print this message



ectophile

Posts: 540
Joined: 17 September 2001

Regardless of how many consumer units there are, the house loads should always be supplied first by the inverter, and then by the grid if the inverter isn't producing enough power.

This is because the inverter monitors the incoming mains supply and sets itself at a slightly higher voltage. Provided there's enough sunlight, the inverter should be pushing current out to the home appliances, and out to the grid.

If you draw it out as a circuit diagram, you'll find it makes little difference how the loads are arranged in consumer units. There are two voltage sources, connected in parallel, and a number of loads connected across the paralleled pair.

However, the inverter will be current limited by how much power the panels are generating. If the load is greater than that, then the inverter's output voltage will drop, and current will be drawn from the grid.

-=-

For any given solar installation, the installer should have a pretty good idea how much power it will generate. There is computer software for that sort of thing, based on panel orientation, latitude, amount of shading and so on.

If the customer is complaining that they aren't saving money, it should be simple to look at the reading on the generation meter to see if the system has been working properly.

If there's been a bad connection, this may have disrupted the operation of the inverter and reduced its output. Inverters are designed to shut down if they detect a fault on the mains supply.

-=-

The effectiveness of the solic immersion control will depend on it being wired correctly. If it can't measure the power flows around the system, it won't know when to turn on. I don't have one of those units (yet), so I don't know how they are supposed to be configured. I would hope that it comes with an instruction manual.

-=-

Is the overheating an arc or a high-resistance joint? If it's the former, then I wouldn't expect any huge explosions anyway - the inverter will limit the current flowing regardless of how low the resistance of the arc itself drops. If it's a HR joint, then it will reduce the current flow, but still get hot in doing so.

-------------------------
S P Barker BSc PhD MIET
 18 June 2013 03:12 PM
User is offline View Users Profile Print this message



jcm256

Posts: 1841
Joined: 01 April 2006

 18 June 2013 03:19 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11295
Joined: 13 August 2003

Where do you fit the RCD protection with micro inverters?

Wouldn't they go on the "grid" end of the a.c. inverter circuit (e.g. at the CU) - just like for a normal PV inverter? Provided the RCD is DP it'll de-reference the inverters output from earth when it trips, so avoiding shocks to earth even if the inverter(s) are still generating.
- Andy.
 18 June 2013 03:24 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11295
Joined: 13 August 2003

the cage clamp is very badly designed. I can see how it could be very easily missed down the side of the cage clamp, although I suspect a good tug test may have identified any termination problems.

It's worse when it happens in the bus-bar side of the MCB I've noticed that some brands now have a flange below the clamp so that the hole below the clamp is closed off when the clamp is raised, stopping conductors getting below the clamp.

- Andy.
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.