A surge protector, also known as a surge suppressor or surge diverter, is a device designed to protect electrical and electronic devices from power surges or spikes or lightning. A power surge is a sudden increase in voltage that can occur due to lightning strikes, power outages, or electrical faults. These voltage spikes can cause damage to sensitive electronic equipment such as computers, televisions, and audio equipment.
A surge protector works by diverting excess voltage away from the protected device and safely grounding it. It contains a metal oxide varistor (MOV) or a gas discharge tube (GDT) that absorbs the excess voltage and directs it to the grounding wire. Surge protectors come in different forms, including power strips, plug-in surge protectors, and whole-house surge protectors.
Problems Caused by Transient (Surge)
Operation of a Surge Protector Device (SPD)
Surge protectors work by diverting excess voltage away from electrical and electronic devices and safely grounding it. When a power surge occurs, the surge protector detects the excess voltage and diverts it to the grounding wire.
It's important to note that surge protectors are not foolproof and cannot protect against all types of power surges. For example, a surge protector may not protect against lightning strikes or other extreme power surges. Therefore, it's important to use surge protectors as part of a comprehensive strategy that includes proper wiring, grounding, and regular maintenance of electrical equipment.
Types of SPD (Surge Protector Device)
1. Type 1 SPD -
Type 1 SPDs are also known as primary surge arresters or service entrance surge protectors. They have a high surge current capacity and can handle large surges and transients, which makes them suitable for protecting against lightning strikes and other high-energy surges.
They are usually installed at the mains distribution board to protect against lightning/transients/surge.
Type 1 - SPD can discharge partial lightning current with a typical waveform 10/350 μs. Usually employs spark gap technology.
Type 1 SPDs are required by many electrical codes and standards for installations where the risk of lightning strikes or other high-energy surges is high. They are an important component of a comprehensive surge protection system for buildings and facilities.
2. Type 2 SPD
Type 2 SPD (Surge Protective Device) is a type of surge protector that is designed to provide secondary protection against power surges and transients in low-voltage power systems. They are typically installed downstream from the service entrance or at the subpanel level to protect against voltage spikes and surges caused by internal electrical events or external surges that make it past the Type 1 SPD.
They have a lower surge current capacity than Type 1 SPDs but are still capable of handling most surges and transients that occur within a building or facility.
Type 2 - SPD can prevent the spread of overvoltages in the electrical installations and protects equipment connected to it. It usually employs metal oxide varistor (MOV) technology and is characterized by an 8/20 μs current wave
Type 2 SPDs are important components of a comprehensive surge protection system for buildings and facilities. They are designed to protect sensitive electronic equipment and systems, such as computers, servers, and telecommunications equipment, from damage caused by power surges and transients. They are also commonly used in residential settings to protect household appliances and electronics.
3. Type 3 SPD
Type 3 SPDs are also known as plug-in surge protectors or point-of-use surge protectors. They have a lower surge current capacity than Type 1 and Type 2 SPDs but are still capable of handling smaller surges and transients that may occur within a specific area or piece of equipment.
Type 3 SPDs are important components of a comprehensive surge protection system for buildings and facilities. They provide an additional layer of protection for sensitive electronic equipment and systems, such as computers, televisions, and other household appliances. They can also be used to protect specialized equipment and systems, such as medical equipment and industrial machinery.
They are characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).
It's worth noting that Type 3 SPDs are often used in combination with Type 2 SPDs, which provide secondary protection for the entire electrical system, and Type 1 SPDs, which provide primary protection at the service entrance. Together, these three types of SPDs form a comprehensive surge protection system that can protect against a wide range of electrical events and surges.
Risk Assessment to Determine If Surge Protector is Required - (BS 7671:2018)
In order to understand whether surge protection is required in cases not fitting those outlined above, a risk assessment should be carried out to determine the calculated risk level. In the event a risk assessment is not undertaken, surge protection is required to be fitted.
The risk level is calculated using the formula outlined in section 443.5 of the wiring regulations (see below).
1. Environmental Factor are give by -
2. Calculate the power cable length (Lp) in km, as shown in figure 44.3 (BS 7671:2018) (if any lengths are unknown, assume that LPAL completes the total length up to 1km)
3. Find the lightning flash density (Ng) from figure 44.2 in the wiring regulations.
You can also see the real life data for any country on this map
4. Put the above values into the formula:
CRL = fenv/(LpxNg) - If CRL < 1000, surge protection is required to be installed.
For Example:
1. Building in an rural environment
Ground flash density (Ng) = 1 Environmental factor (fenv) = 85 Power cable length (Lp) = 2LPAL + LPCL + 0.4LPAH + 0.2LPCH LPAL = assumed 0.6km LPCL = 0.2km LPAH = 0.2km LPCH = unknown = (2 x 0.6) + 0.2 + (0.4 x 0.2) = 1.48 CRL = fenv/(LpxNg) = 85/(1.48 x 1) = 57.4
CRL < 1000, so surge protection shall be installed
Where
- LPAL is the length (km) of low-voltage overhead line
- LPCL is the length (km) of low-voltage underground cable
- LPAH is the length (km) of high-voltage overhead line
- LPCH is the length (km) of high-voltage underground cable
3. Building in suburban environment located in north Cumbria supplied by HV underground cable Ground flash density Ng for north Cumbria = 0.1 (from Figure 05 UK flash density map) Environmental factor fenv = 85 (for suburban environment – see Table 2)
Risk assessment length LP
- LP = 2 LPAL + LPCL + 0.4 LPAH + 0.2 LPCH
- LP = 0.2 x 1
- LP = 0.2
Where:
- LPAL is the length (km) of low-voltage overhead line = 0
- LPAH is the length (km) of high-voltage overhead line = 0
- LPCL is the length (km) of low-voltage underground cable = 0
- LPCH is the length (km) of high-voltage underground cable = 1
Calculated Risk Level (CRL)
- CRL = fenv / (LP × Ng)
- CRL = 85 / (0.2 × 0.1)
- CRL = 4250
In this case, SPD protection is not a requirement as CRL value is greater than 1000.
Example of Installation Reference
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| SPD installed adjacent to consumer unit (source: Beama guide) |
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| Installed inside a consumer unit (Source: Beama guide) |
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| Installation of type 1, 2 and 3 SPD in a TNC-S earthing |
SPD Installation Criteria
1. SPD protection -
BS7671 section 534 requires installed SPDs to be protected against short-circuit through the use of over current protective devices such as Fuse or Circuit Breaker. OEM of SPDs usually provide clear guidance for the selection of the correct ratings of the overcurrent protection device
2. SPD cable length -
Section 534 advises that SPD connecting cables have minimum cross-sectional areas of 16mm2 (copper) for the high energy Type 1 SPD and 4mm2 (copper) for Type 2 and Type 3 SPDs if the cross-sectional area is greater than or equal to 4mm2, or not less than that of the line conductor, where the line conductors have cross-sectional area less than 4mm2.
These cross-sectional area values are based on the surge current that these SPD connecting leads need to carry, not the supply load current. However in the event of a short-circuit due to say the end-of-life condition of the SPD, the connecting leads to the SPD need to be protected by a suitable overcurrent protection device (OCPD).
3. Connection point -
SPDs are to be connected in parallel to the incoming power of the circuit to be protected.
Parallel connected SPDs are passive devices and under normal operation do not draw any load current although a negligible current is drawn if the SPD has electronic status indication.
Design Considerations for SPD protection of equipment
To achieve optimum protection, the following criteria must be taken into consideration while selecting an SPD
- Determine the maximum voltage rating of the equipment
- Calculate the maximum surge current
- Determine the type of SPD
- Choose the SPD with the appropriate voltage and current ratings
- Determine the location of the SPD
1. Determine the maximum voltage rating of the equipment:
This can be found on the data plate of the equipment or in the manufacturers specifications. Make sure to choose an SPD with a maximum voltage rating higher than the equipment
2. Calculate the maximum surge current:
The maximum surge current is the amount of electrical energy that could flow through the SPD during a surge event. This can be calculated using the equation
I = 2 x (kA x km) x S
where,
I = maximum surge current
kA = maximum lightning current that can be expected to flow during surge event
km = location factor
S = building area
3. Determine the type of SPD:
There are three main types of surge protective devices (SPDs):
- Type 1 SPDs are designed to protect against direct lightning strikes. They are typically installed at the main service entrance of a building.
- Type 2 SPDs are designed to protect against indirect lightning strikes and other transient voltage surges. They are typically installed at the distribution panel or subpanels of a building.
- Type 3 SPDs are designed to protect individual electronic devices from transient voltage surges. They are typically installed at the outlet or circuit breaker of the device they are protecting.
4. Choose an SPD with the appropriate Voltage and Current Ratings:
Based on the maximum voltage rating and surge current calculated in steps 1 and 2, choose an SPD that meets or exceeds these ratings.
5. Determine the location of the SPD:
The location of the SPD depends on the type of SPD chose. Type 1 SPDs should be installed at the service entrance, Type 2 SDPs can be installed at the sub-panel or branch circuit level.
Also, a more greater insight is How do I actually determine the kA, km and S value.
For kA - Maximum Lightning Current
1. Determine the lightning flash density: This is the number of lightning strikes per year per square kilometer in the location of the building. You can check above for this information.
2. Determine the lightning current density: This is the amount of current that each lightning strike is expected to carry. For example, for a typical lightning strike, the lightning current density is about 20kA per meter of the strike length.
3. Determine the buildings effective height: This is the height of the building from ground level to the highest point. Effective height takes into account the height of nearby structures and trees that may attract lightning.
4. Calculate the kA value: Once you have the lightning flash density, lightning current density, and building's effective height, you can calculate the require kA value using the following formula:
kA = (1.2 x 10^-4) x flash density x current density x effective height
where 1.2 x 10^-4 is a constant used to convert the units of lightning current density and effective height to the appropriate units.
To determine kM
The location factor, km, is a measure of the likelihood of a lightning strike occurring in a particular location. It is determined by a number of factors, including the height of the building, the materials used to construct the building, and the presence of lightning rods or other lightning protection devices.
The National Fire Protection Association (NFPA) 780 determines the value of km, the location factor, based on a number of factors, including:
- The average number of lightning flashes per square kilometer per year.
- The elevation of the site.
- The terrain surrounding the site.
- The presence of water bodies or other lightning attractors near the site.
The location factor is used to calculate the maximum surge current that can flow through a building during a lightning strike. The higher the location factor, the higher the maximum surge current.
The following table provides a sample of location factors from NFPA 780:
| Location | Location Factor |
|---|---|
| Open country | 1.0 |
| Wooded areas | 0.8 |
| Urban areas | 0.6 |
| Coastal areas | 1.2 |
| Mountains | 1.5 |
It is important to note that these are just sample location factors and the actual location factor for a particular site may vary. It is always best to consult with a qualified electrician or lightning protection specialist to determine the correct location factor for your building.
To determine the value of S (Building Area)
The value of S in lightning protection is the building area in square meters (m²). It is used to calculate the maximum surge current that can flow through a building during a lightning strike.
To determine the value of S, you will need to measure the floor area of your building. You can do this by multiplying the length of the building by the width of the building.
For example, if a building is 20 meters long and 10 meters wide, then the building area would be 200 square meters (20 m x 10 m = 200 m²).
Typical Example of SPD selection
Determine the size and type of SPD to be used to protect a 3 storey building with service incoming voltage of 400V 3phase 4-wire system and maximum current of 150A. Assuming the total surface area of the building is 500m2
Step 1 - Calculate the maximum surge current
I = kA x kM x S
where: I = maximum surge current (kA)
kA = Maximum lightning current that can be expected to flow during a surge event (kA)
km = Location factor (dimensionless)
S = Building area (m2)
kA = we can use an hypothetical value of 20kA
km = If the building is within a costal area we can use the above to determine the location factor of 1.2
S = 500m2 (this can be also be determine by measuring the total square meter area of the building or can be checked from the architectural drawing of the building)
I = kA x kM x S = 20 x 1.2 x 500 = 12,000kA
So, as a general rule the maximum surge current of the SPD must be greater than the calculated surge current I and voltage rating of the SPD can be of the same value as the service entrance voltage.
Hence, for the above scenario, we will select an SPD of 20kA/400V Type 1
Also, we need to consider a good earthing system for the building as the SPD will be connected to earth also so that surge current can flow to general earth and the resistance of this earthing must be around 10 ohms in other to be more effective.
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