Granny charger fire warning

[Malcolm]

Since we purchased our MGZSEV a year ago we have used the MG supplied granny charger plugged into our carport 3pin 13amp socket for most of its charging.

Recently whilst charging the power to the socket failed & on investigation we found that the other 2 sockets on that ring main were also dead.

Eventually we found a burnt out junction box in the roof of the carport which was a professionally built house extension of some 15 years. As far as we know the wiring had been installed by a qualified electrician. The carport circuit was taken off the existing upstairs ring main of 4x13 amp sockets back to an old fashioned 30amp fuse as originally installed some 45 years since.

In theory the circuit should have been ok as 30amps will give 7kW of power, the granny charger taking 2.2kW plus say a hoover at 1kW plus say a fan heater at 2kW total 5.2kW. However there are several factors that make this assumption erroneous. Firstly the granny charger will take 2.2kW continuously for many hours unlike a hoover for 1/2hr & a fan heater on & off. Therefore any heat build up in the circuit will not readily dissipate until charging ceases. Secondly we estimated that the length of wire from the charger socket back to the fuse box was over 100ft or 30mtrs which would give some voltage drop thereby increasing the current required by the charger. Thirdly we don’t know how well or badly the wires were joined in the junction box & at other places.

I show pictures of the burnt junction box & think we were lucky not to have had a serious house fire. There were scorch marks on the nearby wooden floor joist but I think luckily any flames were subdued by the fibreglass insulation.

Fortunately we have another source of supply on a short run of wire from an RCD box which I shall use even after the problem circuit has been repaired.

Please be warned.

 

[Alb]

Perfect example of what many have warned.
Glad the damage was limited.

 

[Jomarkh]

When I got my MG5 I ran a new cable for an outside waterproof socket to plug my granny into for 6 weeks until I got an Ohme EVSE.
Like you say the granny pulls 2.2kW constantly whereas a 3kW heater clicks on and off and probably would not be on for no more then 4-6 hours.

 

[TheArcher]

View attachment 12375View attachment 12376
Since we purchased our MGZSEV a year ago we have used the MG supplied granny charger plugged into our carport 3pin 13amp socket for most of its charging.

Recently whilst charging the power to the socket failed & on investigation we found that the other 2 sockets on that ring main were also dead.

Eventually we found a burnt out junction box in the roof of the carport which was a professionally built house extension of some 15 years. As far as we know the wiring had been installed by a qualified electrician. The carport circuit was taken off the existing upstairs ring main of 4x13 amp sockets back to an old fashioned 30amp fuse as originally installed some 45 years since.

In theory the circuit should have been ok as 30amps will give 7kW of power, the granny charger taking 2.2kW plus say a hoover at 1kW plus say a fan heater at 2kW total 5.2kW. However there are several factors that make this assumption erroneous. Firstly the granny charger will take 2.2kW continuously for many hours unlike a hoover for 1/2hr & a fan heater on & off. Therefore any heat build up in the circuit will not readily dissipate until charging ceases. Secondly we estimated that the length of wire from the charger socket back to the fuse box was over 100ft or 30mtrs which would give some voltage drop thereby increasing the current required by the charger. Thirdly we don’t know how well or badly the wires were joined in the junction box & at other places.

I show pictures of the burnt junction box & think we were lucky not to have had a serious house fire. There were scorch marks on the nearby wooden floor joist but I think luckily any flames were subdued by the fibreglass insulation.

Fortunately we have another source of supply on a short run of wire from an RCD box which I shall use even after the problem circuit has been repaired.

Please be warned.

15 year old socket? Say no more. Glad the damage was limited.

 

[djenson]

I'm not an electrician, but white PVC cable coming out of a junction box going to sockets? Surely that's not right?

Anyway, glad I ran a dedicated run for my granny socket after seeing this.

 

[smokie]

Eventually we found a burnt out junction box in the roof of the carport which was a professionally jerry built house extension of some 15 years.

There, fixed it for you

Seriously, I'm glad it didn't work out far worse for you. I've read about happenings like this when using chargers before - the potential for disaster is huge really, but I bet few people check it properly before use - and I'm not really surprised by that, not everyone understands this stuff and I didn't till I got an EV and joined forums.

 

[Lovemyev]

Thanks for posting and showing the photo's.
Wow ....... A lucky escape there I would say.
Really a bit of a perfect storm, long cable run supplying the car port, spurred off an existing house ring main with no dedicated circuit / RCD protection, cable joints incorporating two J.B.'s in the circuit.
When the car port was originally wired, it was never envisaged that it would be asked to supply / satisfy the constant power demand of a "Granny" charger.
The failure point was likely to be a loose / bad connection in the J.B.(s) .
It's all been said before, but just because a "Granny" unit has a standard 13 Amp three pin plug fitted, does not mean it should be treated like plugging in ANY other low demand three pin appliances / accessories.
Granny charger are ideally powered from a independent / dedicated protected circuit.
Correctly rated supply cable, suitable for the length of the cable run and the conditions it is likely to be exposed too ( internal / external ).

 

[tsedge]

It never ceases to amaze me how many people skimp on a home charger because they can "get away" with a granny and after testing once or twice, declare it safe on the basis that the "plug didn't get too warm".

You should have a dedicated circuit for a granny.

The condition of your home wiring should be checked by an professional before getting an EV.

You should limit the charge rate, ideally 6A but certainly no more than 10A.

Avoid cheap granny chargers that allow 13A charging.

 

[5teep]

Just to add that while 2.2kw is well within spec, 2.2kw for 10 hours straight can build up some serious heat in a junction. Said junction box had been subject to 15 years of heating and cooling from weather, possibly vibration as well and perhaps a screw hadn't been tightened quite enough originally.

 

[Lovemyev]

That “Old School” JB was never designed or intended to be used in an external environment anyway and certainly never intended to have a feed / supply broken into to satisfy the power any higher demands of a Granny unit, when pulling a constant high load on even a fairly regular basis.
In the photo of that older JB it appears to have TWO additional supply cables taken from the original interrupted supply feed from the CU.
The addition of the demand to power the Granny was just an accident waiting to happen in reality.
Termination points are the weakest link in any electrical installation and must be kept to an absolute minimum in my mind.
An incorrectly “made off” & incorrectly tightened / torqued termination point, is the cause of a high resistance.
Heat is generated locally, which then creates the real risk of fire course.
A dedicated trip protected circuit with a supply cable of the correct rating ( taking into account the length of the cable run also ) and only terminated at the CU and then again at the destination point with a good quality socket outlet.
No joints / spurs on route.
Praise should be given for posting these pictures, as it highlights the potential risks of using a high demand item, for long periods of time without the correct electrical wiring systems in place first.
Apart from the great example shown above.
There are two main risk factors here, when used individually, they pose only a small risk of excess heat.
Number 1 Risk : A high demand load item when heat is evident at the live pin of the three pin plug.
This can be witnessed when using a full fast boil 3.0 Kw electric kettle !.
The plug top becomes warm to the touch of the back of your hand, when it has boiled cold water twice quickly in succession.
But it poses little risk, given the shorter period of time in use is small.
Number 2 Risk : The length of time factor, of using that high demand.
When these two factors are combined, the potential for excess heat to be generated is vastly increased.
High load, plus continuous use over a long period of time and heat will be inevitably !.
A wall box is a purpose built unit that is only allowed to be installed by qualified professionals, who will have checked the suitability of your home electricity set up, and earth bonding points BEFORE installing your unit.
This may also involve including your DNO to check on the supply cable and main protection fuse to the property.
The problem with using a Granny lead, is that almost everybody has a three pin socket outlet in their homes etc.
Just because the Granny has a three pin plug on the end of it, does not necessarily mean THAT outlet socket you intend to use constantly is suitable to power a device pulling that load over a long time period.
I guess a question you could ask yourself is this :- “How many other electrical appliances do you have in your home, that demands a constant load of over 2 Kw’s that you intend to plug in and walk away for anything up to 10 hours ish and maybe overnight while you are sleeping” .
This problem / risk is more evident in older properties that have not received a recent electrical upgrade of course.
A good case for encouraging the use of a wall box I think .

 

[Ayoull]

That joint box hasn't been used as designed, as they're only intended to allow for one extra cable to connected to the original cable not two, and the total load capability of the brass connection looking at the size of the cables is only 16A as a 30A Joint box will accommodate a 6mm conductor that joint box wouldn't.

So if you're pulling over 3.6kW through it you've overloaded it.

BS7671 (wiring regulations) permit the jointing of one additional socket outlet to a ring as long as the total demand is not beyond 16A on the spur, and the ring is rated to 32A, you aren't able to spur off of a spur, for this very reason.

Here it's a case of poor workmanship initially as it appears to be a 16A joint box and multiple spurs from a single point.

Something else to also consider but never is in a domestic setting is kVARs the reactance on the network also called wattless power in that it is still power being pulled but as the current and voltage are out of phase it isn't able to do anything but still generates heat.

Every device unless it is purely a resistive load has a powerfactor (pf) in electrical distribution design we like the pf to stay as close to 1.0 (unity) as possible but are happy to accept 0.95 which means 95% of the current is in sync with the voltage, so 95% of the current being pulled is able to do some work, then 5% is lost due to inductance.

Modern worlds modern problems electronics are capacitive loads by nature and have a leading powerfactor so the current leads the voltage which is a worse situation for the network but still cause the same problem of heat without doing any work. The onboard charger of an EV should have powerfactor correction built in to reduce its capacitive reactance on the electrical system but the quality varies massively, so let's say a 10A granny is capable of delivering 2.3kW (but in kVA total power this is 2.875kVA, as it takes into account powerfactor) and this is near to the limit of a 13A plug top at 2.99kW.s

So let's take a joint box designed for 16A which is 3.68kW that is total resistive load at unity powerfactor, but if you had a 16A EV charger which is a capacitive load that has had some powerfactor correction applied by the car that is now 4.6kVA total power, but only 3.68kW is doing anything the other 0.92kW is just producing heat)

So it becomes very complex very quickly and unfortunately it isn't something many domestic electricians will even take into account this is why at the DNO work on kVA because it is total power and not all Watts are equal when powerfactor is involved.

 

[JimD]

Interesting point about the PF of chargers.

I wonder if there's any data out there on the actual measured power factor of real-world granny chargers (while charging an EV).

I'm guessing to get approved for use in the UK they must be close enough to 1 to avoid overloading a correctly installed 13A domestic socket (which the OPs definitely wasn't).

 

[Ayoull]

Interesting point about the PF of chargers.

I wonder if there's any data out there on the actual measured power factor of real-world granny chargers (while charging an EV).

I'm guessing to get approved for use in the UK they must be close enough to 1 to avoid overloading a correctly installed 13A domestic socket (which the OPs definitely wasn't).

I have a shelly EM1 on my incomer monitoring my power usage but also provides pf of the supply, so could plug mine in and see what I get, I know my house in general is quite poor due to alot of electronics in the house for automation purposes.

 

[JimD]

For those baffled by Ayoull's post, here's a handy link:

A closer look at power factor correction - Charged EVs

Power Factor Correction (PFC) used to be something that electronics manufacturers only grudgingly added to a product – usually because of complaints about breakers tripping prematurely, or to meet regulatory requirements in the EU – but these days PFC is showing up in more and more products...

chargedevs.com

I have a shelly EM1 on my incomer monitoring my power usage but also provides pf of the supply, so could plug mine in and see what I get, I know my house in general is quite poor due to alot of electronics in the house for automation purposes.

That would certainly be of interest!

 

[OriginalBigAl]

Many good replies to this thread, so can I add another aspect. When you throw the granny charger in the boot , and go visit friends or relatives and ask if you can use their electricity, you have no idea of their electrical set up and this is a potential risk you,ll need to consider. Especially if you are staying the night and are half way through the third bottle of Chablis.

 

[Ayoull]

Arguably the best plug socket to use is the one built-in to the cooker switch as it should be fed on a dedicated 6mm Twin+Earth cable so no risk of cable overheating but then you have how to get the cable outside I guess

 

[smokie]

Many good replies to this thread, so can I add another aspect. When you throw the granny charger in the boot , and go visit friends or relatives and ask if you can use their electricity, you have no idea of their electrical set up and this is a potential risk you,ll need to consider. Especially if you are staying the night and are half way through the third bottle of Chablis.

... or hooking up at your camp/caravan site, or AirBnB

 

[MG4London]

Arguably the best plug socket to use is the one built-in to the cooker switch as it should be fed on a dedicated 6mm Twin+Earth cable so no risk of cable overheating but then you have how to get the cable outside I guess

This is useful info thanks.

What would be the best extension leads to run from the cooker plug socket?

 

[Gomev]

This is useful info thanks.

What would be the best extension leads to run from the cooker plug socket?

Expensive but probably the best off the shelf.

EV granny charger 13A weatherproof extension lead

High quality 13A weatherproof outdoor waterproof extension leads heavy duty 1.5mm H07RN-F rubber cable. RCD option. IP66. Designed specifically to overcome the issue created by chunkier plugs on electric vehicles (EVs). 2m, 5m, 10m, 15m, 20m and 25m lengths.

toughleads.co.uk

 

[pollenface]

I installed a dedicated circuit with 25a breaker to run our 6-15a charger.