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MDF Audio Main Distribution Frame Series

MDF security unit

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Product Description
Product parameters
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The security unit installed on the distribution frame in the communication field is responsible for protecting communication equipment from damage caused by overvoltage and overcurrent. The security unit is a device inserted into the main distribution frame (MDF) security terminal block to prevent personal and equipment harm from overvoltage and overcurrent. Overvoltage and overcurrent refer to external voltages and currents that exceed the normal operating voltage and current of the equipment. The security unit has developed alongside the advancement of overvoltage and overcurrent protection devices. Currently, there are mainly two types of security units based on device and product structure: gas discharge tube plus PTC, and semiconductor discharge tube plus PTC.

1. Gas discharge tube plus PTC security unit

Working principle:

When there is an external overvoltage, the gas discharge tube discharges, guiding the current from the external line to the ground. If the duration is long, the heat generated by the continuous discharge of the discharge tube melts the thermal fuse, allowing the external line to directly connect to the ground, achieving short-circuit failure protection, while also outputting an alarm signal.

When the line suffers from overcurrent, the PTC resistance rises rapidly, making the line approximate to an open circuit state, controlling the overcurrent phenomenon. After that, when the overcurrent disappears, the PTC automatically recovers.

2. Semiconductor discharge tube plus PTC security unit

The mode of the gas discharge tube is ionization discharge, and the temperature is high after discharge, which may affect the lifespan of the main distribution frame. At the same time, the gas discharge tube may fail due to chronic leakage, resulting in an open circuit state between the external line and the ground line. In this case, the reliability of the security unit's protection decreases, and it may not provide the necessary protection.

The semiconductor discharge tube security unit not only has the basic functions of the gas discharge tube security unit but also has the following characteristics: the semiconductor tube has a low temperature during discharge, making it less likely to cause fire; there is no chronic leakage phenomenon, ensuring good protection reliability of the security unit; the response speed of the semiconductor discharge tube to surge impact voltage can be less than 1ns, providing a fast response.
Characteristic description:

1. DC breakdown voltage: 230V (190V—260V);

2. Pulse breakdown voltage: ≤800V (1KV/US);

3. Pulse current withstand: ≥5KVA (8//20us waveform 10 times);

4. Pulse current withstand: ≥100A (10/1000us waveform 300 times)

5. Pulse current withstand: ≥5A (15—60HZ 5 times);

6. Overcurrent protection performance: Normal temperature resistance R≤20Ω, resistance R between a/b lines ≤2Ω, non-operating current 100mA, 1h non-operating;

7. Failure protection performance: Operating time: ≤15s (AC220V 2.5A);

8. Current limiting characteristics (25℃):

Current (A)

Operating time (s)	Remarks	0.35
≤20	Current limited to 150mA	0.75
≤5	1
≤1.5	2
≤0.5	3
≤0.2	Current limited to 500mA	电流限制至500m A

Basis for this requirement

1.1 YD/T694—1999《General Distribution Frame》

1.2 YD/T940—1999《Overvoltage Protection Semiconductor for Communication Equipment》

1.3 YD/T741—2002《Technical Requirements for Positive Temperature Coefficient (PTC) Thermistors for Overcurrent Protection of Communication Equipment》

1.4 GB/T2828.1—2003/ISO 2859—1:1999《Sampling Inspection Procedures》

Testing items, testing methods, and technical requirements

3.1 Appearance structure, installation dimensions, and appearance inspection

3.1.1 The appearance structure and installation dimensions meet the requirements of the drawings, the welding points are smooth, neat, without defects or cracks, the color meets the requirements, and the colors of all finished products are consistent.

3.1.2 The product should have clear product model and registered trademark markings, which should be secure and correctly positioned.

3.1.3 The packaging box meets the requirements, with clear markings of product name and model; the box contains complete work number, inspection number, and certificate of conformity. Testing instrument: Insulation Resistance Tester

3.2 Insulation Resistance Requirements: The insulation resistance between line a and line b, as well as between line a, line b, and the ground wire, alarm signal line, and other metal parts, should not be less than 1000MΩ under normal atmospheric conditions (temperature 15—35°C, relative humidity 45―75%, atmospheric pressure 86—106Kpa, same below). (After high temperature test, low temperature test, temperature change test, and damp heat test, it should not be less than 100MΩ).

Testing method: The insulation resistance of the test sample should be tested between each tested terminal and all other terminals connected to the shell or mounting plate (i.e., between any two non-conducting terminals and between any terminal and metal fixed parts). The test results should meet the above requirements. Testing instrument: DM-100B digital micro-ohm and micro-volt meter

3.3 Contact Resistance The contact resistance between the security unit's pins and the security arrangement (or security seat, same below) terminal (or spring piece, same below); Requirements: (1) Under normal atmospheric conditions, the contact resistance between two contacting spring pieces should not exceed 7mΩ (after high temperature test, low temperature test, temperature change test, and damp heat test, the increase in contact resistance should not exceed 3mΩ). The security unit should be plugged and unplugged 200 times at a frequency of 10-15 times per minute on the security arrangement, and the increase in contact resistance between the security unit's pins and the security arrangement terminal should not exceed 3mΩ. In addition to meeting the specified contact resistance requirements, the security unit must be easy to plug and unplug, with a good feel.

3.4 Alarm Signal

The alarm signal is clearly visible when the security unit is activated.

3.5 Overvoltage Protection Performance Parameters (Note 1)

a DC breakdown voltage Udc=230±30V

b Insulation resistance: ≥1×10Ω

3.6 Overcurrent Protection Performance

3.6.1 PTC Performance (Note 2)

a Normal temperature resistance ≤22Ω, resistance difference between line a and line b ≤2Ω; (After testing for surge current capacity, power frequency current capacity, power frequency voltage capacity, and various environmental tests, the re-measured value) imported PTC normal temperature resistance 6.4-9.6Ω (8Ω±20%)

b Non-operating current, 110mA for 1 hour without operation, (test voltage 60Vdc, ambient temperature at 40℃)

c Current limiting characteristics (at 25℃), see Table 1

Table 1

Current (A)	Action Time (S)	Remarks
0.35	≤18	Current interruption or limiting to 150mA
0.5	≤10	Current interruption or limiting to 150mA
1	≤1	Current interruption or limiting to 500mA
3	≤0.2	Current interruption or limiting to 500mA

3.6.2 Internal Fuse (Tube) Performance

Safe current: 0.4A for 1 hour without operation

Operating current: 1A > 0.5S operation, 2A ≤ 0.5S operation

3.6.3 External Fuse (Tube) Performance

Safe current: 5A for 1 hour without operation;

Operating current: 10A millisecond-level operation.

3.7 Failure Protection Performance

Testing instrument: NLT-1 type failure protection and alarm testing instrument

When the discharge tube discharge current reaches 2.5Arms, line a or line b in the discharge circuit should be grounded within 15S.

3.8 Plastic Parts Combustion Performance Test, etc.

The quality requirements for plastic parts in the security unit should be in accordance with the factory testing document 'Flame Retardant ABS Material and Plastic Parts General Testing Requirements'.

3.9 Quality Requirements for Plated Parts

The quality requirements for plated parts in the security unit should be in accordance with the factory testing document 'General Testing Requirements for Plated Parts'.

Testing Rules

Edit

The determination methods for the supervision of security units and the implementation details for inspection are as follows:

Sampling is conducted according to GB/T2828.1—2003/ISO 2859—1:1999 'Sampling Procedures for Inspection by Attributes', with a quality level P0=0.25% and an inspection level of Level II. The implementation details for the security unit are shown in Table 2.

Table 2

Serial Number	Inspection Item	Inspection Sample (pcs)	Remarks
1	Appearance	II	Normal Temperature Environment
2	Insulation Resistance	II	After normal temperature, low temperature, high temperature, temperature change, and damp heat tests
3	Contact resistance between the security unit and the terminal block	S-3	After normal temperature, low temperature, high temperature, temperature change, and damp heat tests, after 200 insertions and removals
4	Failure protection performance and alarm function	S-3	Normal Temperature Environment
5	Overvoltage protection performance	II
6	Overcurrent protection performance	II
7	Combustion performance of insulating materials	S-3	Normal temperature environment according to GB5169.7-85
8	Quality requirements for electroplated parts	II	Normal Temperature Environment

Note (1): Other indicators outside the DC breakdown voltage and insulation resistance tests of the discharge tube are conducted when the discharge tube enters the factory and during routine testing of the security unit. The testing procedures are detailed in 'Technical Requirements and Testing Methods for Semiconductor Discharge Tubes'.

Note (2): Indicators outside the 'Overcurrent Protection Performance' specified in this regulation for overcurrent protection devices are conducted when the overcurrent protection equipment enters the factory and during routine testing of the security unit. The testing procedures are detailed in 'Technical Requirements and Testing Methods for Positive Temperature Coefficient (PTC) Thermistors for Overcurrent Protection in Communication Equipment'.

Note (3): High temperature tests, low temperature tests, temperature change tests, and damp heat tests for the security unit are conducted during routine testing.

Key words:

MDF Audio Main Distribution Frame Series

MDF security unit

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