Home » Products » PTC Thermistor » Telecom circuit protection » thermal resistor (MZ2)

Product name: PTC thermal resistor for overload protection of telecom main distribution frame (MDF) MZ2 series

Item: MZ21,MZ24

PTCR for overload protection of telecom main distribution frame (MDF) MZ2 series

Brief Introduction:
Advanced developments in telephony equipment in rencent years have radically altered the protection requirements for both exchange and subscriber equipment.The PTC thermal resistors must protect telephone line circuit against overcurrent,with maybe the following example:
1)surges due to lighting strikes on or near to the line plant;
2)short-term induction of alternating voltage from adjacent power lines of railway systems,usually caused when these lines or systems occur faults;
3)direct contact between telephone lines and power lines.

Product Application:
They provide reversible,self-resetting protection against overcurrents in:
1. Main distribution frame (MDF)
2. Telecom equipment
3. Telephone

Working principle:
To provide good protection under such conditions a PTC thermal resistor is connnected in series with eachline.Usually as secondary protection;However,even with primary line protection(usually a gas discharge tube),the PTC thermal resistor must fulfill severe requirements.Surge pulses of up to 2 KV can occur and in order to withstand short-term power induction the PTC thermal resistor withstand high voltage.If the line has primary protection,a 220V to 300V PTC thermal resistor is adequate.Without primary protection,600V PTC device is necessary.

The restricting overloading protection PTC thermistor is connected in series in the overloading circuit. when the circuit is in the normal working state , the current flowing through the PTC thermal resistor is not high enough to make the temperature exceed the Curie temperature for protection ,and the PTCR is always in the low resistance state. When the circuit breaks down and the circuit current increases a lot, the resistance of restricting overloading protection PTC thermal resistor heating sharply until the circuit is "cut off" relatively to achieve protection purpose. After clearing the circuit trouble, the PTC thermal resistor begins to cool and come back to the state of low resistance, and then the circuit enters normal working condition. When the circuit breaks down next time, the PTC thermal resistor can enter the state of protection again.

1. Automatic protection,can resume steadily;do not need to be reset after protection
2. Contactless,noiseless and flameless;
3. Reacts fast;
4. With a better performance of withstanding voltage,can operate in 110/230V AC (high resistance state) continuously
5. Low rated current
6. Small steady resistance;
7. Narrow resistance tolerance (resistance matching)
8. Satisfy ITU-T K20 K21 requirement.
--Surge current(10/700µs)
--Induction voltage(600V,1A,0.2S)
--Power line contact(230V,15min.)
9.Long stability

Electronic feature:
Rated zero power resistance:
Ambient temperature: 25±2℃,within the allowable tolerance.
Non-trip characteristic:
Ambient temperature: 40±2℃,in still air for 30 minutes, power voltage 60VDC, maximum non-trip current ON for 60 minutes. (R-Rn)/Rn≤50%
Overcurrent trip characteristic:
Relation between trip current to trip time of PTC thermistor.
Over voltage withstanding:
Power voltage: 230 Vrms /250 Vrms, maximum current ON for 30 minutes. 4 hours after restoring in room temperature, retest zero power resistance, ΔR/Rn≤20%.
Over current withstanding:
Power voltage 220Vrms, maximum current 60S ON/600S OFF, cycles: 20, 4 hours after restoring in room temperature, retest zero power resistance, ΔR/Rn≤20%.
Surge current withstanding:
Short circuit wave: 10/1000μs, minimum opening circuit voltage: 1.0KV, peak current:25A, interval time: 3 minutes, cycles: 30,4 hours after restoring in room temperature, retest zero power resistance, ΔR/Rn≤20%.
Induction voltage withstanding (Only for maximum voltage over 600V):
Power voltage: 600 Vrms /650Vrms,initial current 1.1A, 2S ON/600S OFF, cycles: 10, 4 hours after restoring in room temperature, retest zero power resistance, ΔR/Rn≤20%.
Fail model:
Ⅰ. Power voltage: 250Vrms,initial current: 10A.
Ⅱ. Power voltage: 600Vrms ,initial current: 15A
Ⅲ. Power voltage: 650Vrms,initial current: 10A
Energized time 30 minutes, high resistance state or open circuit is allowable. Low resistance or firing is not allowed.
Restore time:
The time necessary for PTC thermal resistor returning to twice Rn. Lower than 60 seconds is required.

Marking of Part Number:

1.Series: PTCR for overcurrent and overload protection of telecom apparatus;
2.Type:1-disc,2-encased,3-pin form-coating;4-pin form non-coating;
3.R25 resistance:55R-55;
4.Resistance tolerance:M+/-20%;
5.Non trip current(25 ℃):070-70mA;
6.Wire shape:S-straight, A-axis formed, U-in-formed;
7.Max. Voltage:250-250V.

Dimension(Unit: mm):

Remarks:Except for the reference size and parameter above,different electrical parameter and encapsulation mode can be custom made per client's requirement.
You can send us required specifications to info@dxmht.com or provide reference samples to our factory office:
And then samples&orders can be custom made per your requirement.