IGE.2 - Lightning protection box?

[Keith]

This is a black (bakelite?) box with, apparently made by AEG, approx 5"x1.5"x2" mounted on one of my roof joists. It has been previously disconnected by BT and replaced with a flimsy modern plastic connection box. I am about to remove it prior to re-painting. I assume the earth wire which trails untidily down the wall to and earthing rod can also be dispensed with. It doesn't appear to be connected to anything else (and presumably shouldn't be in a PME installation). Any advice appreciated.

[Nickthedentist]

Is this the sort with two very long ceramic fuses in it?

[Keith]

There are two sets of clips that look as if they could have held fuses with blade type terminations. There don't appear to be any spark gap devices - possibly just the gap across the insulation to the earth terminal was used?

[Nickthedentist]

See here: http://www.britishtelephones.com/lightng.htm :

Early this century the protection fitted to telephones was discontinued and the protection was fitted externally to the telephone. At both ends of the line fuses and mica protectors were fitted. This involved an earthed protector unit with fuses on the customer premises and in the exchange, fuses, protectors and heat coils. The heat coils were to guard against low voltage contact (250 volts).

These were a liability and needed maintenance after storms. The BPO decided in the 60's that lightning protection could be dispensed with as the cost involved in replacing protection was greater than the damage to apparatus. They replaced fuses with dummies and removed mica protectors. The heat coils were left in place as these did not blow with high voltages.
It must also be remembered that telephone apparatus at this time was still very simple and were fitted with robust components. It was not until semiconductors were introduced that lightning protection was reconsidered.

[Keith]

Thanks Nick. I guess that's what it is. Interesting bit of history - the "heat coils" sound like something out of H.G.Wells!

[russell_w_b]

Originally Posted by Keith

There are two sets of clips that look as if they could have held fuses with blade type terminations. There don't appear to be any spark gap devices - possibly just the gap across the insulation to the earth terminal was used?

Like the one shown, you mean? This was the later type of lightning arrestor, and superseded a larger version that contained tubular fuses, a porcelain base, and a bakelite cover held in place by a knurled nut.

The unit shown has the fuses in place and two slide-in blocks which I take to be some sort of composite VDR-type material discharge-path to earth.

[Keith]

Russell,

Yep - that's the one, although mine had a chunk of the cover broken out, presumably by the "professional" BT engineer when he replaced it. I wondered about the transient protection technology. I guess it was too early for the sealed gas discharge type devices (which would in any case probably have been too costly). Thanks for posting the picture.

[russell_w_b]

Originally Posted by Keith

Russell,

Yep - that's the one, although mine had a chunk of the cover broken out, presumably by the "professional" BT engineer when he replaced it. I wondered about the transient protection technology. I guess it was too early for the sealed gas discharge type devices (which would in any case probably have been too costly). Thanks for posting the picture.

The slide-in protector to which I referred to as some sort of composite VDR device is (as I've just discovered) actually known as a 'Protector Electrode 1A' and consists of two channel-section brass electrodes separated by a perforated insulating film. The moulded surround is actually of a very high insulation resistance!

These slide-in units are interchangeable with the earlier-type slide-in carbon units (of the same shape): 'Protector No: 14', which PE1's superseded. The inner surface of the older carbon units were flat with bevelled edges, with a thin film of anti-dust varnish and a film of insulating varnish. The specified discharge voltage was in the order of 500 - 750V.

The lightning arrestor of the type in my picture does not have heat coils; the older version does. The fuses shown (hexagonal section with a knife-blade) are rated at 2.5A and will operate within 30 seconds of the current reaching a value of 5A. They are known as 'Fuse No: 37/3'.

[Tim]

Yep - that's the one, although mine had a chunk of the cover broken out, presumably by the "professional" BT engineer when he replaced it

Hi Russell
When I used to work on customers faults I came across a few of these, still in place at customers premises. ISTR the Bakelite cover was quite difficult to remove, especially if "glued" in place by paint. I favoured the direct pull rather than a lever off, and found them best gripped, finger and thumb at each end. Even then they could pop off suddenly and even I broke one or two!!

[russell_w_b]

Originally Posted by Tim

Hi Russell
When I used to work on customers faults I came across a few of these, still in place at customers premises. ISTR the Bakelite cover was quite difficult to remove, especially if "glued" in place by paint. I favoured the direct pull rather than a lever off, and found them best gripped, finger and thumb at each end. Even then they could pop off suddenly and even I broke one or two!!

I've seen quite a few of these in my time, though not always for a living... Me being me, I've always 'spotted' them in folks' houses along with the junction boxes and types of telephone, insulators (where present, and of which I have an accumulation of about thirty), dropwires, etc... Well, it's only natural, innit? It was easy to spot which houses had them fitted, as there was a bare copper earth-wire cleated to the wall, signifying either a lightning arrestor or party-line instrument.

The lightning arrestor in my pic is one of several we still have in our crypt at work, where the telephone wires lead out to the field-telephone points at the aerial field and matrix switches. Long-since disused, they not only had to cope with lightning, but the potential (no pun intended) for RF transmitter feeders to fall on them, induced RF currents, pickup...