OVO NEARLY CAUSED A CATASTROPHIC HOUSE FIRE

There is still a technical error here that has not been resolved. You said: 

                    … As part of the safety checks after install, the tails being undersized wasn't picked up which eventually led to an overloaded system. ...

It is completely impossible for undersized tails to cause an overload. The DNO fuse blew because of either a current in excess of the fuse rating or that it was faulty.  Undersized tails cannot cause an increase in current. Undersized tails can only result in overheating and damage to the tails specifically which is why the tail size is increased to prevent damage to them.

To that extent the reason for the overload that caused the fuse to blow has not been identified and still remains a potential problem even if tails are now correctly sized.

One possibility exists. When the tails were 16mm2 the charge point should be set to a maximum of 16A / 3.5kW until the tail size had been increased and the DNO had checked and resized the fuse if necessary. My guess is that the fuse was 60A. To allow the charge point to run at 32A / 7kW the DNO would have to change the fuse to 80/100A and OVO change the tails to 25mm2 between the fuse and the smart meter. Somehow the charge point was set to 32A and along with the other loads in the house exceeded the 60A fuse rating. That's what caused the fuse to blow, not the tails.


Peter

What size was the main fuse that blew?

NG were the first DNO, I think, to stop installing 100A fuses as standard in domestic premises, opting rather for 80A. This still allows for a load of up to 18kVA, enough for both an ordinary EV charger and some heating equipment. 16mm² of copper is quite enough for 80A, especially over the few inches normally between the cut-out and the meter. Which safety provision does your electrician think was violated?

Hi Peter, 

National Grid confirmed the old fuse was 80amp, same as the new one. 

No new appliances have been fitted and nothing has changed. Nothing new in my routine either. I don't understand what could cause the overload apart from the EV being on. 

My electrician did say that the burning smell could be from the 16mm tails overheating. There's no burn marks on any wires. 

Hi Firedog,

The old fuse was 80amp and the new one is the same. Would a 100A be better situated in my situation? 

The burning smell which emitted when the fuse blew was attributed to the tails. Had the tails been 25mm, they would have dealt with the load power suitably. 

As a piece of information not necessarily relevant, the marking on the fuse carrier is not the size of the fuse. It's the rating of the fuse carrier. Mine says 60/80A but it is fitted with a 60A fuse as I'm on a looped supply but houses not looped can also have 60A fuses as standard, particularly on older properties.

​@Firedog You are absolutely correct to say DNOs fit 80A fuses these days as part of the diversity calculation for their substations. It means more customers can have an EVSE fitted without technically having to upgrade the substation. It's a similar diversity calculation as you would have in a consumer unit relative to the DNO fuse rating.

If the DNO fuse blew then the wiring would have taken a bit of hammering and, as you said, even if there are no scorch marks they can smell. Other high power devices likely to cause an issue are 8/9kW electric showers, electric ovens washing machines but this group of devices don't normally take full power for very long periods of time. At 100A an 80A fuse will hang in for as long as four hours (checked information). They are there to prevent short circuits starting a fire in the wiring between the DNO fuse and the consumer unit because under those conditions they will blow very quickly and with a considerable bang.

Peter

https://www.northernpowergrid.com/sites/default/files/assets/Northern%20Powergrid%20LCT%20installer%20stakeholder%20communication%20July%202023.pdf

​@Faz890 Do you happen to have the smart meter data for the event?

It was either a catastophic short or the consumer unit took a huge current for an extended period of time amounting to up to 100kWh of power being consumed over a period of hours given the current and the duration taken to blow the fuse. I’m concerned that you still have a serious, unrectified fault on your system at present as undersized tails was not the cause of the problem.

Peter

Hi Peter,

100KWh is impossible. I have checked my meter data and it shows max 20KWh, that was including my EV being charged. The electrician has seen the wiring and nothing has come up. Apart from the 16mm wiring issue. I also have surge protection unit separate for the EV charger. Nothing was tripped before the outage. 

Undersized tails cannot cause an increase in the current. If anything they reduce the current very slightly due to their slightly higher resistance. There is still no root cause for the fuse blowing. It's as simple as that.

With regards to surge protection they can fail short circuit and can blow the fuse but the indicators will be showing red to say that they have either failed or come to the end of their life. When surge protection works properly they they absorb the spikes by drawing up to 40,000A but the energy in the spike is nowhere near the energy required to blow the fuse by many orders of magnitude.

https://www.youtube.com/watch?v=cZC-fNSX2eg&t=35s

Peter 

The safety issue is still present. If the tails are getting hot, there is a fire risk should they melt. They should have been replaced. OVO still need to rectify the problem. 

Wiring regs say 16mm2 is the minimum size for an 80A cutout. It's only if you have a 100A fuse do you need 25mm2.