Corroded swa gland - garage CU

[Leyland30]

Hi,

Have a garage consumer unit supplied from the house CU via a 6mm 2core swa. I noticed this morning the gland entering the garage CU was loose so opened it up to tighten up the gland lock nut. Unfortunately it appears the gland as it enters the garage cu has corroded and brocken away from the lock nut. It's only been install 3 years and the other glands show no sign of corrosion. I have suspicions as to what has caused it but would like some other views on the matter. Pics attached. Thanks

 

[telectrix]

dissimilar metals in contact caused galvanic corrosion. . the steel rusts. solution is to use brass locknuts, or earthing nuts.

 

[Octopus]

Use Piranha nuts

 

[davesparks]

Switch it off and don't switch it back on until it is fixed.

 

[Leyland30]

That's what I was thinking. As you have probably already established, the swa is utilised as Cpc. As a consequence there is no separate dedicated bonding provided within the garage I.e. Earth rod / nest as this isn't required, however; one of the outgoing ways from the CU serves an external metallic lighting column. Could this accelerate the corrosion?

 

[Wilko]

Electrolysis is speeded up with current flow but it needs to be DC to have effect, if I recall my schoolboy chemistry correctly

 

[Leyland30]

Thanks all, the CU is aluminium and I thought the gland was copper / brass. It's the gland which appears to have failed, in this instance at the point it passes through the Al CU. I can't have this happening again, who knows how long ago it actually failed. I'll investigate Al glands.

 

[Dave OCD]

Are you sure the board is aluminium, looks like a BG 'garage' unit ? I thought these were cast steel but could be wrong.

 

[Leyland30]

I'll get my blue peter magnet on it in the morning.

 

[westward10]

Looks die cast.

 

[davesparks]

Looks die cast.

Yes, but that doesn't help decide what the material is!

 

[Marvo]

What else is connected to the earth bar?

Did you pull the shroud back from the gland? Was there any sign of moisture ingress?

Uncoated brass and nickle-plated brass glands are designed for use with dissimilar metals ie steel armour strands which are galvanised so I don't see how the gland having a galv nut would cause such advanced corrosion in such a short time.

If the CU enclosure material is ferric or steel based then moisture under the gland nut could cause it to rust, especially where the paint might have been cleaned off to ensure good contact for the earth ring. Rust causes expansion (think of windscreens that crack when rust gets under them on a car) which could have cause the gland to split.

It's worth noting that moisture around the gland could be caused from it dripping inside the enclosure but could also be coming from inside the cable sheath by capillary action. so I'd disconnect the armour at both ends and megger it to a known earth, if it fails then start digging to find the cable sheath damage.

 

[timbobelfast]

Classic case of galvanic corrosion........

 

[Leyland30]

Marvo, your onto something. The corrosion is rapid, something is speeding it up. CU is Al.

 

[Richard Burns]

I feel fairly sure the BG IP rated boards are cast alloy (not sure what alloy) not aluminium, the metal does not respond like aluminium and it would not be all that "non combustible" as aluminium is highly reactive (when exposed) and has a low melting point.
A current flowing or induced in the metal of the CU would increase the corrosion rapidly and I would agree with Marvo about the possibility of water in the shroud, though it does look like it is coming from above.