paulr

Hi
Just setting up a dedicated computer room at home for my pc,hi-fi etc.I have a load of 4-plug sockets but none have any fancy protection stuff.Is it safety hype or should i get something.I'll have about 20 plugs coming off one socket.
All low amperage though.

Shy Muppet

I've got all my computer/tv/stereo stuff plugged into surge protectors. We had a massive power surge in my area last year and a lot of people had to replace expensive electrical items (fridge freezers, TVs, etc) as they got fried!

Thankfully, we didn't loose anything

paulr

So you'd just have one in the wall socket and that protects all subsequent plugs/sockets etc.

Shy Muppet

I'm not a sparkie, so I would get advice from one (especially if your equipment was a lot of kerrrrrrrrr ching!), but it seems to be OK. I don't have as many plugs as you have mind.

Harry_Boy

Would suggest getting a UPS with surge protection, so that, if a power outage occurs, you won't lose what you're working on with the PC.

Having a UPS will give you a 'window' that enables you to effect a controlled shutdown of the computer, saving your documents etc.

Sixty-ish quid should get you a decent line interactive UPS with surge protection.

Scoobychick

iTrader: (1)

We lost our ADSL modem last year after lighting struck the pole outside and travelled down the line and fried it We now have a trailing socket with built in surge protection and sockets for the modem to plug in to.

Harry_Boy

This link will tell you all you need to know about surge protection and lightning strikes.... www.aelgroup.co.uk/htm/domestic.htm

Leslie

The point to consider is that a lightning bolt has aready travelled through quite a respectable distance to reach the ground due to its extremely high voltage.

Having got that far, a surge protector or a fusible link is not going to do too much to slow it down should it get into the system!

Les

Harry_Boy

True Les - but a decent TVSS can absorb the energy before it causes damage to critical electronic components.

Graz

I don't think so somehow! There's a hell of a lot of energy in a lightning strike. All these so called "surge protectors" give you in terms of lightning is a shorter path to earth rather than it going through your equipment to get there, however due to the unpredictable nature of the path the charge will take there is no guarantee. I suspect if a direct strike is made on your telephone line, tv aerial or house then it will fry pretty much everything regardless of how many surge protectors you have.

Back to the original question. If you are concerned that your overloading the 13A wall socket then a distribution block with a overload detector / trip and/or current meter may be worthwhile. TV's, hi-fi, computers, etc. are all low current so it should be possible to plug quite a bit in without overloading. Also remember that the plug going into the wall will have a 13A fuse in it, however fuses are not great, it may blow at 13.1A however it's more likely to be at a much greater current than that.

Harry_Boy

Sorry mate, but I don't think you've grasped how a TVSS works.....

Brendan Hughes

Harry, to non-tecchies the link is not very informative except for saying "you need one of these". Care to elaborate? (I understand/guess that a surge might be stopped by a fuse/CB, but lightning may well jump a small gap).

Harry_Boy

Fair point Brendan

I'm going to quote from selected bits from the web link, as I actually co-wrote much of the material.

Lightning protection is based on two basic principles. Firstly, the strike is likely (not certain) to hit a metallic mass linked to a solid ground. Secondly, once the energy is largely directed to ground, Surge Protection devices will afford some protection to sensitive electronic devices.

So, for a complete (and for many, OTT) belt and braces protection regime, it's worth considering installing the following devices:-

1) Minimum of two ground rods (electrodes) at least 10 feet deep
2) Down conductors
3) Connect gutters or other grounded metals as required
4) Air terminals (lightning rods) located within two feet of outside corners of chimney
5) Antenna mast connected to roof conductor
6) Air terminals (lightning rods) spaced 20 feet apart along the ridges and within two feet of ridge ends
7) Dormers protected
8) Roof projections such as weather vanes or satellite dishes should be connected to lightning protection system
9) Zone C surge protection devices installed at main electrical panel or meter
10) Zone A surge protection devices installed at electronics in house

The interesting thing, however, is that the majority of power surges are actually generated internally, rather than from lightning strikes (which takes us off topic, but is nevertheless true).

Transient surge can infiltrate electronic equipment by various routes:

Direct Coupling: caused by lightning strike, or faults on power distribution cables.

Inductive Coupling: caused by electromagnetic field generated when lightning hits tall objects such as trees.

Capacitive Coupling: caused by lightning strikes to building lightning conductors.

Resistive Coupling: caused by ground strikes raising the ground voltage which, in turn, causes large potential differences between earth points.

With regard to lightning strikes, both cloud to cloud and nearby ground strikes produce electrical field voltages of hundreds to thousands of volts per metre. A cloud-to-cloud flash can induce a surge of 7,000 volts per metre in power and/or telephone cables and 70volts per metre a mile away. An incidence of two strikes per square mile per year would seem to be an average figure.

As a result of lightning activity, a power surge of between 10-20,000 volts could reach your property. However the maximum normally considered is 6000V, with currents up to 3,000A appearing at your building main power distribution board.

The impedance of the cabling installed on site limits the amount of current that reaches your equipment. The main low impedance bus bar could carry the full 3000A, but the 30A twin & earth feeding a spur presents a much higher impedance that will limit the current to around 200A for the
same transient.

Inevitably, insulation breakdown in cabling, mcbs etc. will limit the level of transient surge voltage within the building wiring.

The effect of a transient surge on a circuit is not only dependent on size but also where it hits on the AC cycle. A 200V transient appearing on the sine wave at zero crossover will have little effect, but the same transient appearing on the peak of the sine wave will add 200V to the peak value of
the sine wave value.

In this example, the load will be subjected to a voltage of 525Vac. And remember, transient surges can be positive or negative going.

Brendan Hughes

(Note to self - don't ask Harry for simple explanations in future...)

Harry_Boy

Basic idea Brendan is that, by the time the surge is close to your electronic equipment, it should be sufficiently manageable for the TVSS (surge protector) to absorb the excess energy.

Now, do you want me to list the various TVSS types and their relative merits and demerits...??

I can also bore you to death on UPS if you like.....

Cheers, HB.

Brendan Hughes

A-HA!

Translation of all the above - "Surge protectors only really work if you've put other safety devices in earlier up the chain."

Bloody lawyers, paid by the word

Phew! Can go and have lunch now...

Harry_Boy

By the kilo mate....

Scoobychick

iTrader: (1)

So Harry Boy, do I assume correctly that just replacing a normal trailing socket with a surge protected one won't do much good if a direct lightning hit is taken but can offer protection against some surges? Should I buy more surge protectors or not?

I've read all the above info (well, skimmed through it) but am more confused now

Harry_Boy

Err.... Yes and no....

Okay - so a direct lightning strike would be extremely unusual, but would certainly fry whatever equipment you have connected through a conventional surge protector.

The main problem is that not all TVSS devices are the same, and some can withstand higher voltages and/or current surges for longer and at higher levels than other.

As such, the cheapo surge protected 4-way sockets and so on that you often see are next to useless in this situation. They will generally have a far too high a 'clamping' threshold (the level at which they become effective), and may not react fast enough in any event.

Additionally, they are, all too often, 'sacrificial' so you may only obtain one-time protection from them. The next lightning strike you have may well reveal that the so-called protector was not working anyway.

As you say though, they can offer protection against the 80% or so mains-borne surges that are not directly lightning related.

My advice would be to get a decent TVSS installation from someone such as Advance Electronics - they're pretty cheap, and do work well.

Just remember, there are three basic types of TVSS....

The gas discharge tube (GDT)
The metal oxide varistor (MOV)
The silicon avalanche diode (SAD)

While the GDT might be useful at absorbing very high peak voltages, it would have difficulty with controlling voltages that, although much lower, would still
be capable of causing damage to sensitive equipment.

Additionally, in certain circumstances the arc within the tube may not be extinguished once the transient has passed. This effect can distort the normal waveform and, if the circuit can provide enough current, could destroy
the tube.

MOV type voltage surge suppressors are manufactured from zinc oxide fragments, compressed under very high pressure. Although there response time is very swift, they have a non-linear resistance characteristic. In practice, this means that when an MOV is subjected to a large transient current surge, such as a lightning strike, the clamping voltage can increase to a level at which equipment damage could occur.

Furthermore, longer duration current surges, as distinct
from shorter transient peaks, will cause an MOV to
destabilise.

SADs are semiconductors that can respond very rapidly to a transient voltage surge, and can be specified with clamping voltages that range
from a few volts to several hundred. However, because their clamping voltage needs to be as close as possible to the peak value of the mains supply, it is necessary to use a number of SADs, so that the transient energy can be dissipated without sacrificing the device.

Inevitably, this makes SADs more expensive than their MOV counterparts. However, silicon avalanche diodes will not degrade in use, provided their rated capacity is not exceeded.

A combination of SADs and MOVs provides the best solution to the opposing requirements of rapid response to transients and large total current capacity.

Because the SADs react very quickly, they prevent the clamping voltage of the device rising as more current is dissipated by the MOVs. Nevertheless, it is advisable to use more than just a few SAD, to eliminate the potential damage caused by over current before the MOVs take effect.

Most UPS devices incorporate some form of rudimentary surge protection, usually of the MOV type. Better than nothing, but not ideal....

And, in case you're asking, I use three of these....



And yes, I know its a UPS, but it does have basic surge protection, and yes, it's all my PCs seem to want....

Leslie

Harry Boy,

As an experienced Radio Amateur I appreciate and understand the information that you presented to us.

Your reply to my earlier post was not quite right since I was talking about a lightning strike getting into the system in which case it would have the effect that you mention at the beginning of your most recent post.

Yes a direct strike is unusual but certainly not impossible as I can prove form personal experience both in aircraft and at my home.

Les

Harry_Boy

Hi Les,

Thanks for clarifying this. Having re-read your post, I appreciate that you were referring to a direct strike, as distinct from a mains borne transient surge.