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PD Devices - Frequently Asked Questions

Click on the drop down links below to read a frequently asked question:

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Acronyms referred to for Surge Protective Devices?

SPD - Surge Protection Device (The product designed to protect electrical devices from voltage spikes)

TVSS - Transient Voltage Surge Suppressor (Replaced by the acronym SPD)

MCB - Miniature Circuit Breaker

MCCB - Molded Case Circuit Breaker

RCBO - Residual Current Circuit Breaker with Overload

LPS - Lightning Protection System (eg: lightning rod / conductor, earthing wire system, ground rod)

MOV - Metal Oxide Varistor (Technology used within some SPD’s)

SAD - Silicone Avalanche Diode (Technology used within some SPD’s)

GDT - Gas Discharge Tube (Technology used within some SPD’s)

What is a Metal Oxide Varistor (MOV)?

The MOV is a bipolar, non linear resistor, with a symmetrical voltage / current characteristic curve, and a resistance value which decreases as the voltage increases. A semi-conducting device which “clamps” the output at a specified voltage)

What is a Silicon Avalanche Diode (SAD)?

A SAD is semiconductor diode, which avalanches like a thyristor, and folds back to give less dissipation. A device that “clamps” the output at a specified voltage

What is a Gas Discharge Tube (GDT)?

The GDT is a sealed gas filled spark gap, consisting of two metal electrodes, which are normally spaced by a ceramic or glass insulator

What is a Surge or a Spike of power and where does it come from?

Surges or Spikes of power are rapid changes from the normal operating voltage level, these can come from many different sources - from both outside and inside

Are underground cables protected from lightning?

Underground cables do offer greater isolation to the effects of lightning when compared to aerial cables; however they are still subject to induced electromagnetic coupling of energy from nearby ground flashes.

As such, it is good practice to install surge protection on facilities supplied by both, overhead and / or underground, power feeders.

Where is the best place to fit surge protection?

Ideally, protection should always be installed at the main service entrance. This will ensure that externally generated surge energies are routed to earth by the most direct path.

If utilities companies already install SPDs, So why would I want to fit my own?

The protection the utilities provide is there mainly for personal safety to prevent lightning from migrating in on their wires and causing personal injury and to protect down time and expense on their own equipment.

When the utilities enter your property, it is not protected and anything they have fitted downstream provides little protection or no protection for your sensitive electronic equipment.

Phone companies will also add in their network boxes protection again it provides primary protection but does not eliminate the need for secondary protection in your home at the equipment end. If the issue happens within your property this would be your issue and not theirs.

Can I protect equipment that operates on dc?

Protection of equipment connected to direct current (dc) sources or power supplies generally involves installing protection at the alternating current (ac) input to the power supply.

However, there is a growing need for dc protection when it comes to applications such as telecom. When applying an SPD to a dc bus, the SPD needs to be marked and approved for these types of applications.

What about the installation of SPD’s?

Surge protection devices should be installed as close as possible to the equipment that is being protected. So for example, the closest breaker in a main panel to the main incoming switch. Excessive cabling within an installation will mean that the SPD let through voltage (the voltage that does not get stopped by the arrestor) will be too high for the protection required.

Ideally the maximum cable length will be no greater than 250mm with an absolute maximum at 500mm.

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PD Devices Ltd
Unit 1 Old Station Yard
South Brent
TQ10 9AL

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